How do you implement a queue using two stacks?

Use two stacks: inbox for enqueue, outbox for dequeue. Enqueue pushes to inbox (O(1)). Dequeue pops from outbox; if outbox is empty, transfer all elements from inbox (reversing order) then pop. Each element is moved at most twice, giving amortized O(1) per operation. This approach is useful when stack operations are the only primitives available or for certain distributed scenarios.