What about the structure of water allows you to pop popcorn in a microwave oven?
Each kernel of popcorn contains a small amount of water which
evaporates, causing steam, and popping the kernel open. Water
doesn't have a crystaline structure, thus heat causes jumps in
energy levels easier. Using both these facts, we can then infer
about why the structure of water causes popcorn to pop under large
amounts of energy, called microwaves.