Teflon tape is a white or yellow tape readily available for under $2 at any hardware
or plumbing supply
store. It is used to seal threaded
connections so they do not leak, along with plumber's putty
which may also contain teflon
. This makes it particularly useful for anyone engaging in a little home plumbing. Such as changing a water heater
How it works
Teflon is very slippery stuff. Many plumbing connections are threaded, particularly where plumbing lines connect to an appliance, and/or the material used changes. Such as where you change from pipe to hose to connect to your kitchen sink. One side will have a male fitting, which will thread into a female fitting. The idea is to have the threading provide the seal but to avoid gaps large enough for water to pass through, you have to get the thing really darned tight. Sometimes that's possible, other times it isn't, either because you aren't strong enough, or because the material won't survive the required torque. Copper, for example, is pretty soft. Teflon tape is very thin and slippery. Properly applied, it can lubricate the threads, allowing the fitting to be further tightened. In addition, the tightening effect squeezes the tape, which is quite flexible, and it forms a shim that fills any remaining gaps.
How to Use Teflon Tape
The key is to wrap the tape about the male fitting in the direction that it will be screwed in. The easiest way is to view the fitting from the non threaded end, and screw it clockwise into the tape, so that the leading edge is wrapped and the trailing edge of tape will enter the female fitting last. If you wind it wrong, threading in the male fitting will tend to unwrap the tape, and you'll have to do it over. Which will lead to much wailing and gnashing of teeth. You can use it with plumber's putty which serves much the same purpose, and if you chose to paint the putty inside the female fitting
Fresh teflon tape can also be used to repair leaks from threaded joints, disassemble the old joint, add tape and reassemble. Leaky valve stems can often be repaired with a similar teflon cord. The stuff is cheap, it works, and belongs in every homeowner's tool closet.