What disappears as soon as you say its name?
No looking up this is for fun.

hmm, Is it silence or no? I think its silence!

Explanation:

. a function is a relation in which each input has only one output. in the relation , y is a function of x, because for each input x (1, 2, 3, or 0), there is only one output y. x is not a function of y, because the input y = 3 has multiple outputs: x = 1 and x = 2.