Measure out as much water as will equal your final sauce - that is, if you're supposed to reduce the sauce to three cups, measure out three cups of water. Now pour the water into your cooking dish.
Wrap the rubber band around the chopstick. Holding the stick upright, lower it into the center of the pot until it touches the bottom. Now roll the rubber band down the chopstick until it is just above the surface of the water.
Later, as you simmer the sauce, check it against the rubber band on the stick. When the liquid is just below your rubber band, you know that it's reduced as much as it needs to.
Fancy, right?! We've heard these called jury sticks in some places, which seems like a fitting name. Besides sauces, you can use a jury stick for soups, making jams, or any sauce that you need to cook down to a specific amount.
Give it a try!
(Images: Emma Christensen for the Kitchn)