With temperatures below zero, its a perfect time to try out some winter weather experiments.
1) Snow Blower - Take a pot of boiling water - it must be boiling - and out it into a water gun or super soaker. Watch as the water from the gun instantly freezes to ice before your eyes. This is because boiling water actually freezes faster than water at room temperature. Crazy right? This really only works with temps below zero degrees Farenheit. Be careful not to burn yourself with the boiling water.
2) Frozen T-Shirt - Soak a T-shirt completely with water. Leave it out in the snow or just outside overnight. The water in the shirt will freeze, and your shirt will be a frozen block of ice.
Bonus: Fold it when its still cold to make a "paper" airplane... but with a shirt!
Bonus 2: Use a towel instead of a shirt. The towel makes for a good sled.