Subject: Why did my popped corn have higher mass than my kernels?

I tried this experiment w/ my 8th grade to prove the law of conservation 
of mass. Here it is:
Question:
 Will a change of volume affect the mass of matter?

Hypothesis:
 The law of conservation states that matter can neither be 
created nor destryed so it wouldn't change.

Procedure:
1. Measure the mass of an empty graduated cylinder.
2. Measure the mass of an empty beaker.
3. Put 15 ml  of popcorn kernels in cylinder.
4. Measure the mass of cylinder and kernels.
5. Cook kernels in airpopper.
6. Place popcorn into empty beaker.
7. Measure the mass of beaker and popped corn.

Data:
mass of empty cylinder 27g   empty beaker 51 g
mass of kernels & cylinder 30g  beaker & popped corn 61g
mass of kernels        3g       popped corn     10 g
volume of kernels     15 ml      popped corn     325ml

I did this twice and in both experiments the mass of popped corn increased.

The kids and I are confused.  The humidity was normal; it wasn't raining.

I expected the mass to go slightly down because normal kernels contain @ 
14% water. I could have explained that to the children. But could it have 
gained 7g of air from the environment? Help?!?

Re: Why did my popped corn have higher mass than my kernels?