Favorite single function model

[allanp]

So guys and gals, whilst browsing the pneumatic contest entry topic trying to pick who to vote for I noticed a couple pneumatic steam locos. These are two great enteries that have only a single function. Then I looked up and noticed a model I have built hanging on a string that spans the room near the ceiling, and it's this:

http://rebrickable.com/sets/8891-1-f1/cable-car-parts-needed-to-build-not-included-with-book-technic-1991

This is a fantastic little build whose only function is to automatically go back and forth along a piece of string (yes, it does automatically change direction once it reaches an obstruction placed on the string). I highly recommend building it. There are also some really cool GBCs whose only function is to move a ball from one place to another.

So this got me wondering, what other really cool models do you know of (official or MOC) that have only one function?

 

Edited November 7, 2016 by allanp

[Choops]

8045 is nice and simple.  I find it relaxing to just pick up, move, and set down the skid on top of things.

Steve