That’s a good question, and lots of scientists want to know the answer to that. Lizards don’t grow back limbs, but some lizards can grow their tail back if it falls off. Salamanders, which are related to frogs, can totally regrow limbs – I think that’s what you’re referring to.
We don’t know yet how they regrow their limbs and tail. It may be that they have special types of stem cells that humans don’t have that start the regeneration process. Salamanders and lizards may also have blood vessels that can regrow better and quicker than humans’ do.
When lizards lose their tail, it doesn’t grow back exactly the way it was. Their original tail contained bones, but the new tail has no bones, just cartilage. The new tail is often a different colour and has a different pattern to the old one.