A box with a funnel sticking out.
At the opposite ends an elbow spout.
Inside the box some works, no doubt.

The set of x's is used to fill --
food for machine, grist for the mill.
One y for each x the spout will spill.

Set X -- the domain.   That's not too strange.
The rule (the box) effects the change:
x into y.   Set Y is the range.

The concept's clear -- no mental blocks.
Now this may appear a paradox,
but that is the function of the box.