4. A great cold weather activity is to have students form lenses from ice.
Lenses without air bubbles and other flaws are not easy to make but
slightly flawed ice can still get the idea across. A 100-mm ice lens can set
paper on fire. Some people have made ice lenses very carefully and then
been able to use them in cameras and telescopes. (Handle the ice with
gloves so melting is minimal.) This idea could be turned into many projects.