Cylindropuntia bigelovii — Teddy Bear Cholla, Superstition Mountains, Arizona, with saguaro cactus (Carnegiea gigantea) in background.
Update, 7 January 2009: Reader Stephen Early points out in a comment below that the cactus is in fact a teddy bear cholla, C. bigelovii, not a jumping cholla, C. fulgida. I have changed the entry accordingly. By way of apology, all I can say is (a) I don’t really know from cactuses, and (b) I thought our guide called it a jumping cholla. The Wikipedia entry for jumping cholla notes that the term is often applied to chollas in general.