Ok, just wondering if anyone could help me out on this rule.

If following the river, someone makes a bet and everyone calls, and then someone turns over the winning hand, do the people who have losing hands have to show their cards, or can they fold them without ever admitting what they had?

I have seen players fold without showing before, but does the player with the winning hand have the right to ask what the losing player had?

Cheers for your help!