That's cool! Good luck with it :)
You're right, some lenses are just too easy, but I think it's good to practice with them anyway. Less ideal pieces of glass (or like you said, ice, water, etc) are quite a bit tougher!
Here's a story from my high school days.
I went to a special program for outdoor stuff. We crammed all the book stuff into half a year, then had half a year of white water kayaking, ocean kayaking, cycling, mountaineering, kekuli building, cedar basket making, telemark skiing on glaciers, etc.
A major part of this program was firestarting. Anyone who managed to do so without matches or lighter got extra marks.
About 1/8 of the class succeeded, including myself, with the bowdrill. When my younger brothers went through the same program, they also got fire. I'm surprised how few managed to do it. Some didn't try at all, some tried for months and just couldn't figure it out. I saw one girl get a smoldering pile of tinder once, but she had no practice with fire in general, and couldn't keep it going!