The chemical energy stored in the peanut was released and converted into heat energy. The heat energy raised the temperature of the water in the small can.
Try a couple of other experiments using different kinds of peanuts or other kinds of nuts. Try:
 Raw peanuts
 Dry roasted peanuts
 Vacuumpacked peanuts
 Freezedried peanuts
 Try cashew nuts, Brazil nuts, pecans, walnuts or other kinds of nuts. (Do they contain more energy than the peanut? Why or why not?)
You might want to try more than one peanut. You'll need extra needles. Use four or five peanuts to heat the water. Is the temperature four or five times higher?
Energy is measured in a unit called the Btu, which stands for British thermal unit. A Btu is the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. Using math, you can figure out how many Btu are in the one peanut. (The plural or Btu is still Btu, not Btus.)
First you'll need to find out how heavy 1/2 cup of water is. Use a small scale and weigh the small can with nothing in it. Then weigh the can with 1/2 cup of water in it. That will tell you how much the water weighs.
Then, knowing how hot the water was, how many degrees its temperature was raised, you can figure out roughly how many Btu are in the peanut. (PLEASE NOTE: This will be an approximate figure because the entire peanut will not be completely burned...there is still some chemical energy left inside the partially burned peanut. In order to measure the heat energy exactly, you would need to use a sophisticated piece of machinery called a "calorimeter".)
For example: If the water weighed four ounces (1/4 of a pound), one Btu would raise the water temperature 4 degrees Fahrenheit. So, if your water temperature increased by 10 degrees (70 degrees at room temperature to 80 degrees), 10 divided by 4 would mean the peanut contained approximately 2.5 Btu. This is only an example of the math and will not be the same as your calculations.
One Btu equals approximately:
 One bluetip kitchen match
 0.252 kilogram Calories (food calories)
1000 Btu equal approximately:
 One average candy bar (252 kilogram Calories)
 One hour of bicycling
 4/5 of a peanut butter and jelly sandwich
NOTE: You may see Btu defined as 252 calories. These are International Table calories which are equal to 1000 of the "Calories" or "kilocalories" we use for measuring food energy.
