We have a bathroom sink with a broken stopper -- or at least a stopper I've been ineffective at fixing. So I followed up on a months-old resolution and bought an old-fashioned rubber stopper. To cover all bets, I got one that fits a range of drain sizes. And it works great. I run water into the sink, and the imperturbable stopper makes sure it just stays there.
I admit I thought the device was self-explanatory. But Kate pointed out after I'd removed the stopper and left the package just lying around on the kitchen counter that it came with installation instructions. Or "installation instructions," since nothing there really tells you what you need to do with the drain plug to achieve total stopper satisfaction.