Btw, to add on that: A pushdown automata with two separate stacks is as powerful as a turing complete language. And you could interpret a standard finite state

Btw, to add on that: A pushdown automata with two separate stacks is as powerful as a turing complete language. And you could interpret a standard finite state machine as a pushdown automata without any stack. So the number of stacks is what makes it more powerful (although more than 2 stacks does not make it more powerful).