I wear knitted slippers around the house a lot - at least in the winter - and with the recent cold spell we had, my feet got downright cold. I started dreaming about felted inner soles, but as a rule, I won't buy anything that takes special care so I didn't have any yarn that would felt or shrink.
Thrift store to the rescue! In one of my forays, I found a wool sweater marked "Dry clean," which means that it will shrink if I wash it in hot water, which is exactly what I was looking for.... for $2.50.
I think I'll unravel the arms and save the yarn for a future felting project, but felt the rest. There will still be enough to make felted inner soles to last through several winters' of very frugal toasty toes.