Welcome to the forum. Sounds like you picked an interesting project! It seems like the question you're asking us ("Do strawberries lose weight in the fridge after two days?") is something that should be answered by your project, not something that we can tell you without experimenting ourselves. If you have another specific question, could you post again?
In the meantime, here are some thoughts related to your project:
-- You talk about making a graph. If you weighed the strawberries each day, then you can plot strawberry mass on the vertical (y) axis and time in days on the horizontal (x) axis to make a plot of strawberry mass vs. time. Especially if you are comparing strawberries of somewhat different initial masses, a second plot of % mass lost** vs. time would also be helpful. Are you trying to compare the effect of wrapping in different materials (wax paper, aluminum, plastic), or did you wrap each strawberry in all three? (If you're comparing them, your hypothesis should state which ones you think will help the strawberries maintain their mass and why.) If you're comparing the different wrapping materials, you can make a line on the plot for each with a new color or symbol (a new 'series' if you use Excel). If you aren't comparing different packing materials, are you comparing the strawberries kept in the fridge to strawberries kept elsewhere, like on a countertop?
** % mass lost on day N = 100% * (mass on day 0 - mass on day N)/(mass on day 0)
-- I'm not sure what you mean by "subtract a little bit from that". It's probably most convenient to weigh your strawberries without removing them from the packaging, and you definitely wouldn't want to include the mass of the packaging in a % mass lost calculation. However, you should subtract the measured
weight of each strawberry's packaging from its mass for your plot.
-- Since individual strawberries will vary in ways that affect how they spoil (size vs. surface area, mold contamination, areas bruised during shipping), it would be good to try to select strawberries from the same source and of the same size, etc -- keeping those variables constant as much as possible. You can also minimize the effect of these variations on your results by doing multiple 'trials' -- that is, wrap 5 or 10 strawberries in foil, another 5 or 10 in plastic, etc.
-- It seems like you're mainly thinking of water evaporation as the reason behind any loss of mass that you observe. It would be good to keep in mind that there could be other processes, like mold growth or chemical reactions as the strawberry begins to decompose. I think that both of these would result in the release of gases, which would reduce the strawberry's mass. But mold growth might also add mass from gases in the air to the mass of the mold+strawberry, so I'm not sure what the net effect would be. I will look for a biologist to comment on this aspect of your project. For the time being, it would be very helpful to make notes about the appearance of your strawberries each time you take them out to weight them, including the appearance of mold, color changes, pitting, etc. Photos would be great.
-- You mention freshness. Do you have a definition in mind for "freshness"? Is it the same as "least mass lost"?
Here are a few Science Buddies experiments related to your topic, in case they give you any ideas:http://www.sciencebuddies.org/science-f ... p064.shtmlhttp://www.sciencebuddies.org/science-f ... p022.shtml
And the project guide has more info about graphs, controls, hypotheses, and more:http://www.sciencebuddies.org/science-f ... ndex.shtml
Hope that helps,