Take two tin cans, one with the top removed and the other with both ends cut out to form a tube. Turn the closed-bottomed can upside down and push both cans into dense sand. The closed-bottomed can will sink more easily than the open tube. That's weird because if you push the two cans into water, exactly the opposite will happen.

To see why the effect is surprising, first imagine pushing the inverted closed-bottomed can and the tube into water. Naturally, the open-ended tube slips right in. The closed-ended can, however, traps air that creates a huge upward buoyant force as the air is forced down into the water. Trying to submerge the closed can is like trying to dunk a balloon.