"At" or "in": Places [duplicate]

Possible Duplicate:
"Got it at" or "Got it in?"

When referring to a place, when does one use the preposition "at" and when "in" (such as "at school" versus "in school")?

If you are in somewhere, that means you are a member of that institution, in whatever way is reasonable for that place.

If you are at somewhere, you are just physically there.

You don't need to physically be at somewhere to be in it.

In school: You're a student.
At (the) school: You're at school, right now.

In the hospital: You're a patient in a hospital (this requires being *at* a hospital, too).
At the hospital: Your physical location is the hospital.

In the circus: You are a performer who works at the circus.
At the circus: You are attending the circus.

So this general rule exists, but you might say "Isn't this kind of arbitrary?" Why isn't someone "in the hospital" someone who works there? Why isn't someone "in school" someone who works there?

There isn't really a single answer, other than phrases like "in school" and "in the hospital" are common phrases that English speakers have come to memorize with those meanings.

In short, if you're just trying to say the location of someone, stick with at.

If someone is inside something, you need to use in.

He's at the fairground.
He's in the tent.