What is a Lego GBC?

In it's simplest form a Lego GBC (Great Ball Contraption) is a Lego model (module) that moves Lego balls around.

Modules can be simple, or extremely complex.  As long as they pass the balls along to the next module without dropping them, that is all that matters.  They are often compared to a Rube Goldberg machine due to the deliberate complex mechanisms built for the simple purpose of passing balls along.

a simple module

a complex module

Balls enter the module from the left and exit to the right.  You can combine multiple modules together to form a loop, circuit or layout.  The goal is to pass the balls around without dropping any, or having the module jam or break.

Small GBC layout with approx 30 modules