All indoor faucets, including those in bathroom sinks and tubs, contain an aerator which minimizes splashing, mixes in air and provides a final screening step.   These should be removed and cleaned typically once or twice a year.    There are several different kinds of aerators ,  including hidden or cache aerators that have external (male) threads that can be screwed into and out of the faucet outlet by using a faucet aerator key like these.  

The most common hidden/cache aerator sizes in the US are 16.5, 18.5, 21.5 and 24 mm.   My home has three faucets with 16.5 mm hidden aerators.     These tools can be printed and used to remove hidden/cache aerators of those sizes.        (I suspect these sizes are used in other countries, would appreciate any feedback on whats used globally).

Note that many indoor faucets use other styles of aerators, check yours carefully by comparing to the pictures in the links here.  

I printed these in ASA using 25% infill and 6 walls/perimeters.     Other materials will work, PETG, ABS, even PLA, but if your aerators have not been removed for several years, they may be difficult to remove initially, and you'll have  better chance of success with ASA or ABS.    If you remove and clean the aerators every year, even PLA should work ok.     I would print a couple of extras to have on hand, eventually the very small plastic tabs on any printed or store purchased aerator key will break.

For comparison, these links shows similar items available at Home Depot and Amazon.