Wow! My kids are loving the unit on magnets! I'm starting to mix in a few thermal energy experiments as well. I've got a super easy one! This teaches the kids that there are smaller particles within the water and their movement correlates to their temperature.
First, put a clear glass (I use my science beakers) of water in the refrigerator or add some ice and let it melt.
Second, boil some water and put in another glass that is the same shape/size.
When you slip a drop of food coloring in each one, they will disperse at different speeds. The students can visually recognize that the hot water's particles are zooming around, while the colder particles are slowly drifting to the bottom- or as one student said today, "cold is like old people!". I bet she'll never forget which moves fast and which moves slow! Try it!