8
xkcd #3244: Pullback Drive
(imgs.xkcd.com)
Title text:
"How does the spring not run out almost immediately?" "We pull it back REALLY far."
Transcript:
Transcript will show once it’s been added to explainxkcd.com
Source: https://xkcd.com/3244/
Put another pullback motor in but backwards and you’ve got yourself infinite energy