Enzyme analogy for honors bio? Can't use lock and key?

I can't use the lock and key because my teacher told us that one as an example so we could understand what she wanted so any help? I thought of a usb being plugged into a computer but i don't really like that one. I have also thought of plugging my iphone into the charger but i don't really think that is an analogy. Help Please! It is due tomorrow and it was just assigned today so no i haven't just been putting it off.

Thank you to all that answer.

Added (1). Thanks. I was just about to scrap the iphone idea but then i was looking at stuff for the iphone 5 and that has a different charger so i guess it would work now? Haha i hope so: P I think i will try that one and i'm sure i will hear other kids answers in my class so i will get lots of ideas. And i really like the puzzle piece one, that will be my back up.

USB and iphone charger models would not work. Any USB will fit into any USB computer slot, and any iphone charger would fit into ANY iphone. Don't use that model. The point is that only a certain key will fit into the lock, so it Can't BE what you said. How about:
- puzzle pieces, any puzzle piece will not fit another
-nail and screwdriver, a certain screwdriver needs to be used to screw in a certain nail.
-lightbulb and lamp, you can't put a 150 watt bulb in a 100 watt lamp, bad things will occur
-automobile and fuel, you can't put diesel in a gas powered car.a certain fuel is required for a certain automobile

Those are just things I thought of off the top of my head, best of luck!