Heat the can for 2 or 3 minutes, at least until water vapor comes steadily out of the opening
Once you are sure the can is full of very hot water vapor grasp it carefully in the tongs and quickly submerge the top few cm of the can in the ice water
If you have sealed the can in the water and heated it sufficiently it should be crushed immediately and spectacularly by atmospheric pressure; it will also partially fill with water
Notes About What's Going On
The can fills with steam and pushes cold air out; the water vapor is the steam condensing as it exits the can
Effectively, this process removes gas molecules from the volume of the can and after the steam has filled it, the number of molecules is a fraction of what was there prior to heating
The steam condenses rapidly upon immersion of the can in the ice water; when gases become liquids they reduce greatly in volume and the gas pressure inside the can drops suddenly; also, gases contract upon cooling
The sudden drop in gas pressure inside the can (due to volume contraction upon cooling and due to steam condensation), and the fact that it is sealed off from the atmosphere by the water, causes the can to be crushed by atmospheric pressure
Atmospheric pressure is about 14.7 pounds per square inch or 1.03 kilograms per square centimeter
Note: 1 mol of H2O as a liquid has a volume of about 18 mL. One mole of water as a gas at 100°C and 1 atm has a volume of 30.6 L (30,600 mL). That’s a difference of a factor of 1,700!!
Safety
This demonstration is safe enough for anyone with a bit of common sense to do.
The can and the steam you create in it are very hot, use common caution
The heat source should be carefully monitored and not allowed to burn/stay hot after you are done