Kryptonite is a glowing green mineral that emits a potent radiation that weakens Kryptonians in its vicinity. It is also known as the remains of Superman's home planet, Krypton. Batman keeps it in an "overwhelmingly impenetrable" vault in the Batcave. Robin used it to incapacitate the mind-controlled Superman at the Watchtower. It appeared to affect Superboy, who was restraining Superman, to a lesser extent.