Add the ice and salt to the can.
Leave undisturbed for 15 mins.
Add more ice as it melts down.
The ice melts and starts to fill the can with water. The salt lowers the freezing point of the water, so the water can get below 0 degrees centigrade, making the bottom of the can next to the water very cold.
Frost (ice) forms on the bottom of the outside of the can where it is coldest, and dew (water) is on the top of the outside of the can (where there is no cold water on the inside).
There is always water in the air. When it is cold, what happens to the water in the air? It turns to a liquid and makes water drops (like the top of your can). See wet grass in the morning even though it has not been raining. Called dew. (Show image).
When it is really cold, what happens to the water in the air? It freezes into ice (like at the bottom of your can). This happens on the grass too - sometimes the grass is white in the morning. It is called frost. (Show image).
Note that this activity does not work so well in a very warm classroom.
Does not work in a glass nearly as well (for me).