When you are painting trim around the house with a small brush, it's handy to have a small container of paint with you. Home improvement stores sell paint cups, but they all cost a few dollars or more. I have one of these, but sometimes, my little assistant wants to help and I need another small paint cup. If you have Solo cups available, these work great. But, these aren't always around. You could use paper cups, but I prefer not to since you never know when they will leak through and break on you. Most of the time, I have bottled water readily available. When you have an empty bottle, use a utility knife or scissors to cut it in half. Wipe it mostly dry on the inside and add your paint. Voila! Instant free paint cup. When you are done, you can either rinse it out and recycle it, or throw it out. I'm divided on what is worse for the environment: washing the paint down the drain, but recycling the plastic or throwing the whole thing into garbage destined for the landfill.
This can also work anytime you need a small disposable container. I have used this to mix 2 part epoxy. You can estimate the ratios by adding to equal heights on the bottle. Most have equally spaced ridges that make it easy to approximate. Or, if you want to be more precise, you can place the cup on a scale and weigh out your epoxy. When you are done, you can toss it out.