Here's a preview of what we'll be playing with: Submarines.
I will do Sink/Float with the kiddos first. I collected a bunch of items and will have three separate bins. One bin for "sinks", one for "float", and the third bin is for "depends". They will "hypothesize" which items will sink and float. Then they will test their hypotheses. I'm hoping to use "depends" as the conversation starter about water weighing down objects. Sponges and cotton balls are two examples of things that float until saturated with water. We will then test some water bottles (also a "depends") to see if we can make them neutrally buoyant. Then we'll do a modified submarine lesson from the link above...main difference is attaching the straw with clay instead of poking a hole through the cap.
If time, we'll play with the weight (pennies) and see if we can get the submarine to hover (the pennies + water actually weigh it down so it sinks fully).
1. What makes the submarine sink? (weight = water + pennies)
2. What makes the submarine surface? (air, it's lighter than water)
3. What makes the submarine hover in the middle of the water? (the perfect balance of water+air)
I did something similar with the 3rd-5th graders at a summer camp I taught at 6 years ago. They used whatever recyclables they brought in from home for their submarines and had a few lead-up activities to where they were really understanding buoyancy. The difference: I let them design their own submarines. We also used balloons to help the submarines surface.
***Note: many water bottles are now more "eco-friendly." They do not make ideal submarines anymore since they crumple easily when a rubberband is wrapped around them. However 20-oz diet soda bottles work wonderfully! I better get to drinking!