Cactus

A Cactus (plural Cacti) is a block in Minecraft that is found naturally in Desert Biomes. Like in real life, cacti hurts the player. The most efficient way to harvest cacti is to destroy the bottom block, because entities (thrown items and mobs) take damage. Then, if you want to replant the cactus, place it on the ground where the other cactus was.

Growing
Cacti can only grow on sand, up to a maximum of 3 blocks. Cacti cannot be placed next to another block. When harvesting the bottom, the whole cactus breaks, since it is an entity. Cacti doesn't need light to grow.

As a crafting ingredient
In a furnace; Top Slot: Cactus. Bottom Slot: Fuel. = 1 cactus green in the output.

Farming
Cacti needs to be placed on sand, but cannot be placed next to another block. When mining, destroy the bottom cactus block so all the cacti falls and doesn't get destroyed.