reacter
nounUK /riˈæktə/
US /riˈæktər/
A person or thing that acts back when something happens.
The little reacter jumped when the loud balloon popped.
OTHER MEANINGS
2. A substance or device that changes because of another substance or force.
The chemical reacter turned blue when the two liquids mixed.
3. Someone who shows how they feel right away.
As a quick reacter, she smiled the moment she saw the puppy.
4. A part of a machine that moves in answer to a signal.
The reacter in the toy car made it turn when the remote was pressed.