Cactus

Cactus is a block that damages any player or mob that comes in contact with.

Breaking
Cactus can be broken using any tool or by hand.

Natural generation
Cactus commonly spawns on sand in a desert.

Usage
Cactus can only be placed on sand or another cactus block. If any block besides air or another cactus is placed directly adjacent to a cactus block then the cactus will break.

Farming
Cactus will grow by 1 block vertically every so often up to 3 blocks tall and does not need light to grow. Bone meal cannot be used on cactus.