How do you unlock the joker in arkham asylum?

in the last little episode of the joker's part. both he and harley are inside of bells. you must build a weird little thing out of legos, and when stepped on, it bangs the bell and they both jump onto a platform. joker's helicopter comes and tries to shoot you, and you have to build one of those water squirters. spray the helicopter, and when it dies, either harley or joker will jump down and attack you. your instincs will tell you what to do from there