if you ever have or ever will do a small plumbing project, you should know that water “shut off” valves don’t always “shut off” due to age and mineral buildup. they merely “greatly slow down” but leave water slowly dripping. these caps will help and they’re super cheap. just show this photo to the associate at the home store. you can use them with teflon tape or (in my case, i used) a small rubber o-ring to make a seal while disconnecting a faucet or toilet water supply.