Whenever a player casts an artifact, instant, or sorcery spell, you create a Mutagen token. (It's an artifact with "{1}, {T}, Sacrifice this token: Put a +1/+1 counter on target creature. Activate only as a sorcery.")
If you steal mutagen that doesn't exist from aliens that also don't exist, it's not really theft. More like creative writing!
![April O'Neil, Human Element [Teenage Mutant Ninja Turtles Commander]](http://www.cardmerchanttakapuna.co.nz/cdn/shop/files/7ba7e281-0868-5ffe-addc-af9398431005.png?v=1782335857&width=1445)