Fire

Fire is a block that will destroy itself over time, if not on Netherrack or a Magma Block. It will clone itself to nearby spaces, and it can destroy flammable blocks like wood or wool. It is possible to walk through it, but at the cost of some damage. If you stay inside the fire block, it will burn you and continue to damage you until you cool off. In Minecraft: Story Mode, there is a similar block called a Flame Pillar.