That is when they weigh less than the water they displace they go up. When they weigh more they go down. If they weigh exactly the same they float right where they are.
Submarines vary their weight by adjusting the amount of air in the ballast tanks.
You can use a pen cap and a bit of modeling clay to build a submarine that goes up or down as the amount of air in it's ballast tank shrinks and enlarges. The opening to the hollow portion of the pen cap should be facing down. The hollow portion is your sub's ballast tank. It now has neutral buoyancy. The sub should float just below the neck of the bottle.