How Does the Two-String Box Work with Strings and Corks?

[soccerstarz37]

So My Teacher Posed this Problem, And It Really Bugs Me That I Can't Figure It Out...
You have a closed shoebox with two strings, with corks attached, protruding from either ends. When you pull on the right cork 60cm of string can be pulled out. When you pull on the left cork, the right string goes in, and 6cm of string can be pulled out. How is this possible, what is inside the box? Please explain, so that I can make a similar model!

 

[mgb_phys]

2 Wheels 1 axle