Sink or Float
I will really and truly rep for sink and float as one of the top science activities for preschool and kindergarten kids. Lots of hands-on work, opportunity for exploration, and oh yes, I've seriously had 3 year olds stick with this for 50 minutes. Gather a bunch of small household stuff -- cork, beads, small toys, aluminum foil, rocks, pens, really anything goes here. If you're doing this at home, just let the kids pick what they want to test.
First, the kids need sheets labelled sink or float for sorting. This is a great chance to practice letters, too. With older kids, it's a great chance to get them to make a hypothesis, by asking them to first sort the items by whether they think they sink or float. Remember, it don't matter if they're right or wrong, it just matters that they are thinking. Ask why they think what they think, but don't try to tell them HOW to think. Preschoolers are already great little scientists.
Now we're ready to test -- and go crazy. Even 3 year olds do well with sorting at this phase. We can talk about why we think different things sink or float, and again, it matters less that the logic is correct than that there is some thought, and you can suggest tests to confirm ideas. I've had kids think certain things float because they're warm, so then we try ice cubes. You get the idea.
The final step, and most kids get to this themselves, is to make boats, and see what they can put into the boat. Can they make sinking things float by putting them into their boats? Can they add sails?
See our results below.