If your toilet is about to overflow, quickly remove the tank lid, reach down and close the flush valve. It covers the tennis ball-sized opening in the bottom of the tank. This keeps the bowl from overflowing while you clear the obstruction.To test your toilet, remove the tank lid and add food colouring to the water in the tank. Check the bowl in 10 or 15 minutes. If you see colour, the valve may be leaking. This should be done every few years. Most toilet leaks are silent.If your toilet ball float is leaking and filling with water, take off the float by unscrewing it, empty it, then screw it back on and wrap it in a plastic bag. This will tide you over until you can get a new one.