The magician invites a spectator from the audience to inspect individual loose links of a chain. The spectator is to ensure they are solid and there are no gaps or moving pieces.
The rings are dropped into a metal cup one by one. Next, the magician pours the unlinked pieces into the spectator's hand -- the spectator is asked to cover the pieces with her other hand. Magically, as the spectator opens her hand, the pieces of chain have linked themselves into ONE SOLID CHAIN!
Easy to do, and best of all, it creates great reactions.