A Duplo elevator operated with yarn.

Owen and Johnny made a working Lego elevator operated with a primitive yarn pulley.

Here's a 10 second video with Johnny pulling the string and Ezekiel shouting, "Stop, Boy, stop!"

The second video, this time with Owen working the lift, is pretty staid in comparison.

Here are some photos of the construction details:
 

First, the tower without the elevator attached yet. It needed to be fairly strong, as Duplo towers go, in order to withstand the string-pulling that came later. The Mega Block is there so that the elevator doesn't attach itself to the bottom plate every time it lands. A piece of cardboard could serve the same purpose.