Science Fair help please

I would recommend the following:

1) Same size for each piece of fruit.  For instance 1" x 1" x 1".  (watermelon should be in there too!)

2) Take their weight at hour 0.  These weights will most likely differ.

3) Take the weight every hour for 10 hours or whatever you determine.

4) Get the final weight.

With these numbers you can determine the rate of dehydration hourly.  Also, you can assess the dehydration rate and the final weight as a percentage of initial weight.

For instance, the lemon lost 60% weight in 10 hours.  The orange lost 70% in 10 hours.  And so on.

Also the rate may be interesting too.  You could make a graph with the x-axis being hour 0,1,2, etc and the y axis being % body weight.  I would imagine you would get a curve, showing most weight loss in the first couple hours.

Anyway, just some of my ideas.

Be sure that the environment and measuring is consistent for all samples.  

PS.  Another option would be to see if size makes a difference.  So 1/4 of a lemon vs 1/4 of an orange vs 1/4 of a grapefruit.  That would be a interesting comparison, especially for dehydration rates.

Use the whole fruit (your 2nd option).  If you only use portions, you might not get the same ratio of water.

Side note: this is why you have to precisely define your goal.  Are you including the whole fruit, including peel and seeds?  Or are you just looking at the edible part?  If it's the latter then you could do a cross section like Joe said.

Get the total weight of one at the start, then re-weigh after dehydrating.

I don't know what the rules of the project are.  But if you're time crunched, you could cut them in half first then use a fruit squeezer.  Pour the juice through a strainer and set it aside.  Then put the peels and pulp in the dehydrator for a bit.  Weigh those solids when it's done.  Maybe not as precise as the dehydrator alone but it should be close.

Just make sure you go through the same process for each piece of fruit, no matter what kind it is.

Definitely!  And good suggestion using the whole fruit, too.

My suggestion is to choose which ever way you think would best determine "which fruit has the most water."  (it might be helpful to define what 'most water' means).

The wrong method with a decent rationale behind WHY you used it is more insightful than choosing the right method and not explaining why.

I say, this conundrum is a great way to teach your daughter to have an inquisitive mind.

The beauty of percentages is that is doesn't matter your starting point, it is the ratio of change that matters.  So, you can use whole fruits, but the trick will be to make sure each fruit is "fully dehydrated".  Then you can just use before and after weights to figure out water loss by fruit and compare that to the starting weight to get percentage of water by fruit.  I would do some research on dehydration times to make sure you dehydrate each fruit very close to 100%.

I read the question a couple times.  The answer is Grapefruit.  See the reason for my answer below.

BUT, I would hope the goal is to learn about scientific method, significant figures, and dimensional analysis.  In that case they need to do the experiment.  These are all tasks that are lacking in today's engineering graduates, and there have been some spectacular failures as a result, see Mars Climate Orbiter, 1998. 

I would think that if the question is which fruit has the highest percentage of water by weight, they would need to use the whole fruit.  Size doesn't matter since they're going by percentage.

The trick though will be how to tell if they're really at 0% water,  I guess when the weights stop changing.

See below for my reasoning on Grapefruit:

Grapefruit is the largest of the three fruits, and the skin thickness could be less, but probably not more, than the other two, depending on the types.

^^ This... by taking the percentage difference between the 1" cube and the dehydrated 1" cube you find the one that contains the greatest percentage of water.  By using whole fruit you introduce too many variables including skin thickness and skin permability (some fruits allow more water to escape via the skin, some less)

I agree with Jacasa, turn it over to your daughter.  While her approach might not be the same as what YOU would choose, if it is supported and she uses scientific rigor to conclude her hypothesis, she wins! 

If she uses the whole fruit, she'll have to do a different comparison and get different results than if she uses equal size pieces of fruit.  As long as her analysis is consistent with the method she chooses, she has done a good job. 

We, as adults, are inclined to see only one approach as the "right" approach where children can imagine lots of approaches with varying degrees of "right." 

I disagree with comparing 1"x1"x1" cubes.  In doing that, you're introducting an additional variable that you cannot measure.  You wouldn't know how much juice was lost from your cube during the cutting.  I would assume that the fruit with the largest carpels would lose more liquid during cutting prior to you getting your wet weight.  It might be a fairly negligible amount, but then again this is science.  Nothing is negligible.

I think you need to start with the whole fruit and work in percentages.  One thing you could do to speed things up is after weighing the fruit, dice it up and dehydrate the pieces.  Then weigh all the dried pieces as one mass. 

No, you are comparing which fruit loses the MOST water.  So even from samples to sample of the same fruit you'll have different amounts of water when you start.  But if you measure the CHANGE over time in the same conditions you'll have which fruit has the most percentage change and hence lost the most water per unit volume.

In your first paragraph you say you can't cut it because you'll lose the water but in your second you say to cut it up... ???

The question was what fruit has the highest percentage of water.  The percentage of water in the fruit is a factor of two things, the natural (wet) weight and the dry weight.  The change over time in the same conditions is irrelevant.  You need to allow each fruit to dehydrate long enough to lose all of its liquid. 

Losing any of the liquid prior to measuring your wet weight would skew your results.  This is what would happen if you tried to cut up and measure a 1" cube of fruit.  You'll already have lost liquid that drained from the sliced carpels, so your cube is not truly representative of the natural state of the fruit.  And (my hypothesis is that) the fruit with the largest carpels would experience a greater loss of liquid than the fruit with the smallest prior to you getting your wet weight.  Again, it's probably a negligible amount and may not be enough to measure on whatever scale the OP has, but I still don't think it's the proper way to go about it.  Think about it, cut up a cube of grapefruit and you'll have some juice sitting on your cutting board.  How can you account for that juice?

And I did not contradict myself.  I said that cutting the fruit will cause you to lose an unknown amount of liquid.  Prior to getting your initial wet weight, this is bad.  After you have your initial weight, it's fine.  If you weigh the whole fruit first, then it doesn't matter how you go about removing the liquid.  Cut it up and even strain it if you want before you put it in the oven, as long as you take measures to retain all of the solids. 

One think I would encourage your girls to do is not just to focus on the procedure, but to then ask the 'so what' question.  Learning how to carry out an experiment is important, and a lot of times fun, but the best science starts with a problem you want to know the answer to.  Once you know the orange/lemon/grapefruit has the highest water content, encourage your girls to ask why?  Why has the plant evolved this way?  What advantage over other plants could it be getting from this?  If they're all similar in water content why could this be?  The great thing is there's no wrong answer here, just a hypothesis they can maybe test next year in the 7th grade science fair.   

Thank you all!  That's why I love BT - I can get help on anything.  I told my DD that I asked on here and she was like why in the world would you ask triathletes about a science fair?  I told her you guys were smart.

Anyway, you are right, I should let the girls decide what to do.  I am sure the teacher will be happy that they are learning how to do the background research, write up the papers, do the experiment and present their results.  It is so hard for me not to help as this transition to middle school has been terrible (not a great school and some terrible teachers - we're looking at private now.)  I did have to help her with her bibliography because she had never done one, I am sure I was helping too much with that already!

The math that tcj103 mentioned is a really good lesson although probably a little ahead of what they've learned in 6th grade.

Surface area of a sphere = 4 * pi * r^2
Volume of a sphere = 4 * pi * r^3 / 3

(yeah I know a lemon isn't a true sphere, but close enough for the point to still be valid)

Even if the kids haven't learned this math yet, you can use those formulas to draw a graph of surface area and volume for a few different radius values (say, 3 through 6).  They'll see how the volume quickly outpaces the surface area as the object gets larger.

In the experiment the water ties in to the volume.  The peel to the surface area.  The kids should be able to take a good guess at the outcome of the experiment after they understand the graph.

Applied science is a really good way to get kids to give "boring math" another chance

One additional piece of advice is to have a discussion with your DD NOW about not comparing her project with those of her classmates.  She may be rattled when she shows up with what she has done and starts comparing it to the stuff her classmates turn in with a lot of "help" from their parents.  I attended a few of these, and it usually took about 10-15 seconds to know whether the student or the parents did the project.  As you've witnessed here, there are different approaches with equal validity, so the hypothesis and method she chooses is valid even if a classmate (or parent) disagrees.  Hopefully, the teacher is receptive to multiple approaches (not always true). 

The problem with using whole fruit, although an ok possibility, is that dehydration times would be much longer, and therefore may not have been conducive to the time frame available for a science fair project.  

Also, the rinds are going to be different consistencies and different thicknesses.  Therefore making an even comparison of fruit dehydration would be more difficult.  

By making it 1 x 1 x 1 (just as an example), you are equalizing your sample sizes and not giving an 'advantage' to one fruit because of its larger size or because of rind thickness.  Every fruit is not on the same playing field.  For instance, a piece of fruit with a larger diameter (grapefruit vs lemon) would clearly have a longer dehydration time, just based on size alone.

When making statistical or quantitative comparisons it's important that all the variables are equalized to make any kind of 'fair' analysis.  Of course if the whole point of the project is to do whole fruits then go ahead.  But those factors mentioned above would have to be considered at the beginning as a possible 'bias' against certain fruit.

By comparing just fruit pieces to fruit pieces you get a better fruit to fruit analysis without any outer advantages.

PS.  I know I'm a BT forum person, but I do have a BS in Statistics and worked at a Masters level in Quality Analysis/Control.  I'm no expert - just my humble opinion.  As someone said earlier, there's no real right or wrong, just what is perceived to be better a procedural approach.