How Can I make an object sink than float without touching it?

In my science class where doing a subject on Submarines were learning how a Submarine works. Can anyone help me I need to know how to make an object sink and float. I need it to sink first than come back up on its own.How Can I make an object sink than float without touching it?
The trick is to include some baking powder that will react with water to produce gasses. Capture these gasses to make your ';submarine'; rise to the surface.





Many years ago I remember a Cap'n Crunch plastic submarine toy that would sink, rise to the surface, then slighly tip to one side and ';burp'; a bubble before sinking again. This would continue until the baking powder was all consumed.





You want something that just barely sinks to begin with.How Can I make an object sink than float without touching it?
That's a tough one.





Perhaps get it close to neutral density, than connect a small weight on the bottom of the boat, with some sort of mechanical trigger so that when the boat hits bottom the weight is released, and the boat will go up.
I had to do this once; look at the source
you would need to build some sort of balast