Spouts, pitchers, batter scoops and other tools make it easy to pour a liquid into a tiny receptacle. But you don't need to spend on those. Just grab a waxed cardboard milk carton, clean it out and you're good to go.
Food52's Alice Medrich has even used this little trick to fill 100 shot glasses with mousse. Here's what you do:
Open the entire top of the carton — not just the spout — by pulling it apart. Wash and dry it, then fill with your mixture. Leaving the spout open, clip the top of the container back together using a large binder clip. Pour carefully, with the index finger of your free hand poised across the spout, and your eye on the container below. Cut off the flow with your finger as soon as the glass or container is filled to the desired level.
Medrich advises against using staples to clip the top, since one could fall into the food, and because you'd want to reuse the carton anyway.
Check out the full post for other great tricks to use instead of buying specialised kitchen tools.