An easy science experiment
2 paper cups, 2 small glasses, water, pin, ice cubes. Optional - food colouring
Make a tiny pin hole in the middle of the bottom of the paper cups, ensuring the holes are the same size. Stand the paper cups inside the tops of the glasses (you need to be able to see the water dripping from the paper cups into the glasses). Pour cold water into one paper cup until it's half full and add some ice to make sure it's really cold. Then add hot water to the second paper cup until it's half full. Watch as the water drips into the two glasses. Do you see any difference?
If the holes are the same size, you'll see that the hot water leaks faster than the cold water. If the cold water is cold enough, it may not leak at all. This is because the molecules in hot water move faster than in cold water. The faster they move, the easier it is for them to slip past each other. That is why hot water is more likely to leak than cold.
If you want to demonstrate molecules, put hot water in one glass and cold water in the other, and add two drops of food colouring to each. Time how long it takes for each of the colours to spread in the water. Molecules make the colours spread.