Whoops is an Advanced Powercard in Bloons Card Storm. It removes all Ammo from the selected Monkey. It requires a target in order to play it, and can target a Monkey on either side. It can target a Monkey that is already reloading, but this will do nothing but spend the card.
Whoops can be used to disarm an enemy Defender so more damage can get to the opponent. It can also be used to deplete a Factory with the Temporary keyword, especially Corvus Academy of Arts; the target will be removed on the next turn of the owner.
If the opponent is trying to use your Defender to activate a Bloon with an On Popped effect, and you can't spread out all of your Defender's damage such that it doesn't pop that Bloon before emptying all of their Ammo, you can use Whoops on that Defender to prevent them from popping that Bloon.
Whoops is practically useless against non-Defenders with 1 delay, because they'll regain ammo on the next turn anyway, unlike Stunned which blocks for a full turn. However, it works well against a Monkey with 2 or more delay.