Mr. R.’s World of Math

So here’s a story you’re probably not going to believe, but it’s true. I promise you! At least I remember it happening…

It was a long time ago, when I was teaching in a school that was a bit strange, and a bit peculiar (but not nearly as peculiar and strange as the school you’re in right now!)

As I remember, it was a cold, dark morning, and I came into my classroom like I always did. Everything was perfectly normal and there was no way I could have imagined what was going to happen to me and my students that very strange day.

It was science time, and my students were doing chemistry experiments. I warned the students that if they mixed the two chemicals we were using improperly, it could be dangerous.

Hannah didn’t listen. She mixed the chemicals improperly and a blue gas rose out of her test tube and filled the entire classroom. I knew the gas wasn’t hazardous, but we all had to evacuate the room for 5 minutes while the smoke cleared. Luckily we were all wearing our safety goggles.

When we came back in, everything seemed normal. But everything wasn’t. We still had 20 minutes left for science class, but first I told they students they were going to have a five-minute snack break.

“Make sure you keep the food away from the chemicals,” I said as I was passing out little bags of cheddar popcorn to the students.
“Not these again!” complained Matthew.
“Why do we always have the same snacks for snack? Little popcorn, teeny potato chips, tiny rice cakes, mini boxes of raisins, itsy-bitsy cheese crackers. Why is everything little, and why can’t we get something good like ice cream or cupcakes at least once!” said Eva.
While Eva complained, I tossed the last bag of popcorn to Jake.
Then something strange happened.
The snack bag didn’t come down into Jake’s waiting hands. It just floated in mid-air.
None of us could believe our eyes.
“Jake, your snack bag is floating!” screamed Carlos.
“That’s impossible,” I said, “unless gravity has stopped working.”
I double checked to make sure nothing else in the classroom was floating, and everything else was still where it should’ve been, so I figured gravity was still with us.  
“Maybe my bag has less popcorn then the other bags, so it’s lighter and can float,” said Jake. 

Actually Jake’s bag had 562 pieces of popcorn in it, and the other bags all had 721 pieces. What was the difference in the number of pieces of popcorn between Jake’s bag and the other bags?