Diving pen cap

We made a pen cap that dives on command. We took a pen cap, attached a string loop around it and added four paper clips to the loop as weights. We then filled a big soda bottle with water until it was almost completely full. We dropped the pen cap into the bottle and screwed the top on tight. When we squeezed the middle of the bottle, the pen cap sank because the air bubble inside it was compressed. When we let go, the air bubble expanded again and the cap floated back to the top. It took a little practice to get the weighting and water level right. When the cap capsized, causing the air bubble to escape and the cap to sink to the bottom, we used a magnet to drag the paper clips and cap to the top so we could reach in and pull it out without emptying the bottle.