The Chollas (on the left) are nasty: if you touch them, they seem to be jumping at you because they stick to you.

This is one of many cholla cacti also known as "jumping cactus". it is notorious for very loose joint attachment, then the joints attach to hapless by-passers with the slightest brush. See more information here: en.wikipedia.org/wiki/Cylindropuntia_fulgida.