As an additional cost to cast this spell, tap any number of untapped creatures you control.
Burn at the Stake deals damage to any target equal to three times the number of creatures tapped this way.
5/1/2012 Players may not respond to the paying of costs. Once Burn at the Stake has been announced and put on the stack, no one can try to respond by tapping any creatures.
5/1/2012 The amount of damage Burn at the Stake deals won’t change if any creatures tapped to cast it become untapped or leave the battlefield before it resolves.
5/1/2012 All creatures tapped this way become tapped at the same time.