My favorite experiment to talk about is the Hershey-Chase experiment. This was done in the 1950s by Alfred Hershey and Martha Chase. They ingeniously used bacteriophages to demonstrate that DNA, not protein, was responsible for the inheritance of traits. I like it because it is elegant and requires very little background knowledge to appreciate. Students like it because they get it, but they also limit because bacteriophages look like spaceships. It’s a great anecdote for reinforcing nature of science, and using it to intro DNA allows students to draw the conclusion that DNA transmits traits for themselves using the same data as Hershey and Chase.
My favorite experiment to do with kids is the calorimeter lab. They set food on fire and then calculate the amount of calories in the different food samples based on temperature change. It’s memorable and allows for productive conversations about energy content in different types of foods and molecules. I like that it dramatically visualizes chemical energy.