See what we did for other letters in the Alphabet in Simple Science.
E for Evaporate
I felt that to be able to understand the concept of evaporation, he needed a little background on Solids, Liquids, and Gases. I started by reading him States of Matter which does a pretty good job of explaining them clearly. We read it multiple times (in fact, I like the series so much, I'm planning on getting some of the other books). Then we went around the house and talked about whether things were solids, liquids, or gases. The bathroom had a lot of great liquids to explore (soap, lotion, mouthwash, toothpaste, etc). He's getting pretty good at identifying the differences.
Plates or cups, water
We used a twisty dropper to squirt just a little bit of water on two plates. We put one plate inside and one plate outside. Then we let them sit for a couple of days. We came back every once in a while to make observations.
Evaporation is kind of a tough concept for a little guy. When we first put the water out, I asked him what he thought would happen to it. He wasn't sure, so I asked if the puddles would get bigger, smaller, or stay the same. Every time we came back, I asked him what he noticed. When he said the puddle was smaller or had disappeared, I asked where he thought it had gone. He was not so sure, so I explained that it evaporated, and that meant it turned into a gas. I do not like that I had to do so much explaining for him with this investigation, but I figure it was still good early exposure to the concepts.
The water spot had gotten smaller after a day or so. The plan was to let it sit on the counter until it all evaporated, until Xander sipped up the water from the plate...