defined by function

I was sitting in my chair this morning when my nose started to run and itch.

Silas was playing on the floor in the living room so I said "Quick Silas, get me a tissue!"

Silas seems to have a runny/snotty nose all the time, so we have him pretty well trained to find and fetch the tissue box, and he's pretty good about letting us wipe his nose and will even blow it himself when he's really snotty.

Well, he jumped up and ran into the kitchen (passing the tissue box, which was on the couch) and brought me back a cloth napkin from the napkin basket.

I said "No no Silas, get me a tissue buddy".

He looked at me quizzically for a second, unfolded the cloth napkin and honked his nose into it quite loudly as if to demonstrate the point that it would serve the same function and I should stop being picky.

Then he patted me on the leg, stuffed the snotty napkin into my hand and went back to playing on the floor.

That's my boy.