• A tall glass or plastic container
• Some cooking oil
• Ice (try adding some food colouring to make it easier to see).

• Fill the glass with oil.
• Drop a block of ice into the glass. The ice should float in the middle of the oil.
• Let the child watch the ice as it melts.

Ice and cooking oil have almost the same density so a block of ice put  into oil will barely move. As the water melts, it turns into denser, liquid water. The water tends to stick to the ice for a while. If there is enough liquid water on the ice, then the density of the ice and water together is greater than the oil, so they will sink. Once the of water falls off the ice, the ice float.