Example of Closed Set in R^2: Help Needed!

[pantin]

help! 'set' question

Give an example in the set notation of a CLOSED set S in R^2 such that the closure of int S is not equal to S.

I originally used the set

s={ (x,y) : 0 <x^2+y^2<1}
but I just noticed it's not closed set!
...

can anyone give me an example?

Thanks!

 

[HallsofIvy]

No, but I can suggest something to try: if S is a finite set of points, what is its interior? What is the closure of that?