I don't know anyone who hasn't heard of the classic experiment where you add vinegar to baking soda and watch the chemical reaction (which produced carbon dioxide bubbles). For L and E (3 years old this month), the scientific details of this chemical reaction is beyond their level of comprehension. However, this fun experiment is a great way to reinforce the concepts of the scientific process and observation...
First I let the girls spread baking soda on their plates using a spoon. I asked them to describe the baking soda (what color is it? how does it feel?). We've been talking about states of matter lately, so I asked if the baking soda was a solid or a liquid (this is a tricky one for the little kids!)
Then we compared water and vinegar (is anything different about their appearance? what do they smell like?). I invited the girls to drop water on the baking soda and observe what happens. Then they dropped vinegar on the baking soda and observed the reaction (what does it look like? what does it sound like? what do you think is happening?). The girls noted that little bubbles formed and it made a fizzing sound. We talked about how the carbon dioxide in the bubbles was a gas. Later I added some food coloring to the vinegar to make it more fun. I let the girls play around with this experiment for a while. They made some great observations - for example, if they drop the vinegar onto a place where they had previously dropped it, the reaction was much smaller (or didn't happen at all).
In the future, we can repeat this experiment using some variations (like seeing if we can blow up a balloon with the carbon dioxide from the reaction, or trying different acids like lemon juice to see if the reaction is different). These little preschool science experiments are really instilling a love for science in my children, which I think is wonderful.