However the character was then shown to be immune to kryptonite, which is radioactively poisonous to all Kryptonians. Believing that Mon-El has been deceiving him, Superboy tries to trick him with a fake kryptonite meteor made of lead, which turns out to be Mon-El's one weakness. Furthermore, exposure to lead is irreversibly fatal to Daxamites, which Mon-El explains, having regained his memory. Guilt-ridden over inadvertently poisoning him, Superboy saves his life by sending him to the extradimensional Phantom Zone, where he would be able to observe things happening in the outside world, but as a phantom would not age and his lead poisoning would not progress.