Penny, Sheldon and others in “Pilot”

I'm a Sagittarius, which probably tells you way more than you need to know...

Yes... it tells us that you participate in the mass cultural delusion that the sun's apparent position relative to arbitrarily defined constellations at the time of your birth somehow affects your personality.

Participate in the what?

What he means is that wouldn't be his first guess.

Yeah, a lot of people think I'm a water sign.