Things that you can use in place of other things but not vice versa.

Ironically enough, that sounds like something a Darwin Award candidate would do.

Well, if all your iced tea glasses are dirty, you can certainly put a cold drink in a good-sized coffee mug. However, I would not recommend putting hot coffee into an iced tea glass!

You can use a book as a doorstop, but you cannot read a doorstop like a book.

You can use vinegar to clean glass, but i would not use Windex to make salad dressing.

You can use a large magnet in place of a glass paper weight to hold your papers on a desk top. You can’t use the glass paper weight to stick papers to the fridge door.

[George Carlin]You can prick your finger, but you’d better not finger your prick![/Carlin]

Unless it has got many vegetables

But can you eat spongmonkeys with a fork? Maybe for them, you need a spork. . .

You can stuff your bra with cotton balls, but it would be pretty hard to apply antiseptic to a cut with a pair of falsies.

Actually, any tool can be a hammer, but a hammer cannot be any tool.
Except, of course, a hammer.