It’s very likely you have several cans of old paint in your garage or shed. Leftover paint tends to be something you keep, just in case you need it again. As a result, the old paint gets put away, sometimes for years. However, old paint often can be reused, if it’s stored correctly. Even if it doesn’t look like you can use it, there are a couple things you can do to “resurrect” it.
Check systematically through all your old paint cans. Remove the lids using the end of a screwdriver and check the contents. Separate the cans into two sections: reusable paint and throw-away paint. For example, if the paint can contains less than 1 inch of paint, it's probably dry and not reusable. Alternatively, if the can has more than 1 inch and isn't dry, you may be able to reuse it once you've tested it.
Remove the "skin" from old oil-based paint. Pull the skin to one side, using a stick. Lift it out of the can and dispose of it. Notice a layer of oil on top of the paint. Use the stir stick to mix the oil into the paint until the two amalgamate.
Mix old water-based paints, using a stir stick. Look for a layer of watery paint on the top. Stir this into the harder, bottom paint. If the paint is too thick, add a little water until it reaches the desired consistency. Reuse old water-based paints as a primer or undercoat, if desired.
Mix the contents of old paint cans together to make new colors. Create a new color by mixing two or more colors. For example, mix blue and yellow to produce green; or red and green to get brown. Experiment by mixing a small amount of paint together until you get your desired color.