Dutch expression from the youth to express that you have to think about something, mostly creative, before you can provide an answer. The dutch word "klei" means clay, which is a matter often used in creative classes, like art class ( clay modeling)
The expression evolved to taking a dump, because it was considered funny to suggest having a brainstorm session, while sitting on the toilet doing number two.
If someone asks you which color he/she should paint her wall in order for it to fit the other colors in the room, and you don't know the answer right away you can ' klei' about it. ( dutch: laten we erover kleien')
If you want to express you're going to the toilet, to take a dump, you could say ' I'm going to model some clay for a moment' ( dutch: ik ga even kleien)