Can you bend a king mattress?

It depends on how much you intend to bend the mattress and the type of mattress. Many modern mattress manufacturers offer king- and (less commonly) queen-sized mattresses with hinges in their frames to allow them to be temporarily bent to, say be moved to the second story of a home through a tight stairwell. With smaller mattress sizes, you really should not bend the mattress beyond 10 or 15 degress (and even that is stretching it for some brands).