The Weight of Water
You can take one pound of water, put it in a pressure cooker, set it on a fire, the water gets hot and starts to boil.
The pot still weighs the same.
Open the top and a HUGE steam cloud goes up. The pot is now dry, and ONE POUND LIGHTER! That one pound of water became one pound of steam that floated away. It's kind of weird to think that one pound of anything can have "weight" at one point, then be so light that it floats away. It changed it's state of being to be something totally different.
It'd be cool if we could do that. Oh wait... we can.