No You Don't

Gotcha — If an opponent says "I cast" and then
refers to a spell they control, you may say
"Gotcha!" and sacrifice No You Don't. When
you do, counter that spell.
Gotcha — If an opponent says "I control" and
then refers to a permanent they control, you
may say "Gotcha!" and sacrifice No You Don't.
When you do, that opponent sacrifices that
permanent.