This impossible bolt demonstrates a technique called "co-processing", where one can modify a 3D printed part as it is being printed. It's also a fun 3D printed object to trick your friends with!

This only has the file for the bolt and uses a 1/2-13 nut.
Check out my blog: https://printeraction.wordpress.com/
and my vlog about it here: https://www.youtube.com/watch?v=QGW6zMLpge8&feature=youtu.be

FOR METRIC VERSION: http://www.thingiverse.com/thing:688972

When the bolt is about halfway printed, pause the print and screw a 1/2-13 nut onto the bolt.

The MakerBot Replicator 2s lower their build plates when you select "change filament". If you have a different printer, it would be usefule to print two bolts, and screw the nut onto one of them while paused over the other.