Mox Blades

{t}: Clash with an opponent. The winner
adds one mana of any color to their mana
pool. If the winner does not control Mox
Blades, until their next end step, they do
not lose this mana as steps and phases end.
(Each clashing player reveals the top card
of their library, then puts that card on the
top or bottom. A player wins if their card
had a higher mana value.)