We all know that if an object's density is less than the liquid, it will float. If it is greater than the liquid, it will sink.

Take two beakers of water and drop a blue plastic cylinder in each. They will sink to the bottom (Fig.1).  After a minute one will raise up and float (Fig.2).  Using tweezers, swap the two cylinders (Fig. 3).  After a minute the floating one will sink, and the sunk one will float (Fig.4).  Swap them again and wait a minute.

The plastic has properties such that it will shrink in cold water (increasing its density) and sink. In hot water it will expand (decreasing its density) and float.

To start you need a beaker with ice-cold water. Have a 1000 ml plastic beaker of ice and add water. Wait a few minutes for the water to chill.

Prepare boiling water in the electric kettle while you wait. 

Strain the ice water into one of the 600 ml beakers and pour the boiling water into the other 600 ml beaker.

If you prepare these out of sight and then move right to the demonstration at the beginning of the lecture, some students could be puzzled for a while.