Abstract: A trap-forming device that includes a hollow body having an upstream orifice situated in a bottom portion of the body for communicating with an upstream length of fluid evacuation pipework, and a downstream orifice situated in the bottom portion of the body for communicating with a downstream length of the evacuation pipework. The device also includes a retainer defining an overflow edge and cooperating with a portion of the upstream length of pipework to retain a barrier-forming quantity of fluid, and a control displacement mechanism for displacing the retainer between an active position and retracted position. The trap-forming device may be connected to a toilet bowl.