I know that groupoid refers to an algebraic structure with a binary operation. The only necessary condition is closure.
However, I couldn't find any easy-to-understand example of a groupoid which is not a semigroup. I did come across some examples of (certain type of) matrices but then matrix multiplication is always associative (thus making it a semi-group).
So, could someone please provide me an example of a groupoid which isn't a semigroup?