grant problem

Hello All,
i have a problem here.

i am ony giving usage grant on a function in my schema  to another user.
Then how come he's been able to execute the function in the following way?

select ins.validate('nyname','mypassword');

But if i don't give usage grant and straight way give execute grant on
the function, the user gets an error:- permission denied for schema
ins.

i want to give the user execute grant on a particular funtion in my
schema and not all functions.

How can i do that?

Thanks & Regards,
Raj.