[very beginner] function

"[email protected] [ocaml_beginners]" <[email protected]>
Newsgroups gmane.comp.lang.ocaml.beginners
Message-ID <[email protected]> 
I understand this

# function x -> x*x ;;

But don't understand this

# (function x -> x * x) 5 ;;

It is a call of the function? Why do I have to put the complete
function-definition again there?
Isn't there a simpler way? Something like

# x (5) ;;
- : int = 25
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