Spirit is willing, but the wheel is stuck
Spirit is literally dragging a wheel through Mars' dusty soil since the right front wheel has stopped working. And, based on the picture above, it looks like all that dragging has scraped off some of the wheel treads. Must be pretty abrasive soil.
As we all know, the Mars rovers have lasted waaaay longer than anyone expected, and they're still hoping there's life in the old girl yet. NASA engineers are using test rovers to figure out how to get around on Mars with only five wheels.
I just can never get over the fact that these awesome pictures are from the surface of another planet! I always wonder if people will eventually get to Mars and how they'll feel about running into one of our old rovers or crashed spaceships. I sort of imagine it would give you a weird time-traveller feeling. Because it's not like the equipment is going to get all rusty or covered with vines. They'll probably look exactly the same in 500 years as they do now. Maybe covered with dust, though.