...the success or failure of the date would
be based on the biochemical REACTION...
The Big Bang Theory Season 1, Episode 3
...or drinking coffee while driving
reduces one's REACTION time...
The Big Bang Theory Season 2, Episode 5
For every action,
there's an equal and opposite REACTION.
The Big Bang Theory Season 2, Episode 22
This isn't the REACTION
I expected
The Big Bang Theory Season 5, Episode 5