Take some water and let it freeze (without bubbles) and then carefully shape it into an ice lens in order to focus the suns energy on dry tinder to make fire. Good stuff to know if your out in the cold and need to make fire. Not that you would have a lot of time to do this if your only chance of lighting a fire was this method. I think you would freeze solid before you got the technique down. Perhaps this is the sort of thing that should be practiced before you need to use it. However, even as ice has a lower refraction index than glass with some time and practice you could build a simple refracting telescope out of this. A few ice lenses and some way of mounting them would be about all you would need. How cool would it be to be able to look at the magnified craters of the moon with an ice telescope?