It happens, whether it’s a guest or a family member, you know… If it does, and there’s no plunger, first remove the top of the tank and push down on the flapper valve to stop the water’s flow. If it looks rusty, don’t worry, it’s sanitary. Now, lift the ball to the left of the valve and hold up so you can trick the tank into thinking it’s fill of water. Lastly, turn the valves under the toilet clockwise to cut off the water. Once the water has ceased, try to find the source of the clog. You may need the assistance of a plumber, but at least you stopped a potential flood.