We have a black box function f that takes in a bit (0 or 1) and returns a bit. We aim to determine if the function is constant (always outputs the same bit) or balanced (outputs 0 and 1 equally). We can do this by comparing f(0) to f(1). Classical computers require two function calls to make this determination, while quantum computers using Deutsch's algorithm can do it in one call. Quantum functions must be reversible, meaning each output uniquely identifies the input. Techniques exist to make non-reversible gates reversible in quantum computing.