This simple marshmallow activity can show air pressure at nearly any grade level! You can model it with younger students or have older students do it using more complex tools and more advanced terminology.
As you can see, the marshmallow on the left have been expanded while the ones on the right are being compressed. (Or turning "all wrinkly" as one of the 5th graders squealed!) The students loved creating this simple vacuum. I really thought our 5th graders would get bored with this in 2-3 minutes, but ten minutes later they were still going strong!
I also found these neat You Tube video from a middle or high school science lab. It would be appropriate to show to any age group. EXPANDING MARSHMALLOW!
This video gives a great explanation of what is happening! (Way better than I could via this blog!) Everyday Science: What Pressure Does
What cool science experiments are a must do in your classroom each year?