Like a shot
Definition 1 of 2
Pro Tip 1/3Immediate reaction
Use it when someone reacts or responds at once without hesitation. 
She accepted the job offer like a shot.
Pro Tip 2/3Colloquial expression
It's informal and best suited for casual conversations, not formal writing. 
He left the room like a shot after the phone rang.
Pro Tip 3/3Surprised by speed
Indicates someone did something so fast it might surprise you. 
The dog ran off like a shot when it saw the cat.