1) He was a chemist and fell in to a vat of chemical which made him what he is
2) He quit his job as a chemist to be a comedian and his life just goes south real fast and goes crazy and tries to rob his old work place and ends up in a vat of chemical
There are some more but it usually ends with him in a vat of chemicals. He sometimes in the disguise of Red Hood or associated with him in all the stories.