...caused by relative motion between
the SOURCE of the wave and the observer.
The Big Bang Theory Season 1, Episode 6
Leonard, it is a SOURCE of great pain
to me and my family...
The Big Bang Theory Season 4, Episode 18
as a SOURCE of information
The Big Bang Theory Season 11, Episode 16
I know that money can
be a SOURCE of friction
The Big Bang Theory Season 12, Episode 3
But not so deep as to introduce
a new SOURCE of friction."
The Big Bang Theory Season 2, Episode 5