PBS
Pbs Learning Media: Buoyancy Brainteasers: Buoyancy Question
This interactive brainteaser from the NOVA: "Voyage of Doom" Web site challenges you to figure out what causes an object to sink.
PBS
Pbs: Buoyancy Basics
NOVA explores the basic principle of buoyancy. Also included at this site are some interesting brain-teasers related to the topic.