There are times when you may use only part of a can of motor oil - such as when you top off the oil in your car's engine before a long trip.

What do you do with the leftover oil? It's still good. It's really too expensive to throw away. But how do you keep dust and dirt from getting inside the hole (or holes) you've made in the top of the can?

One of the best ways is to save the plastic lid off a small coffee can (which is exactly the same size as the top of an oil can), or the plastic lid off any other food container that happens to be the same size. Many coffee cans, shortening cans and other containers have plastic lids that slip over the top precisely. This method not only keeps dirt out of the oil, but will keep the oil from spilling if the can gets knocked over.

Plastic wrap, foil - even sandwich bags fitted over the top of the can and held by a rubber band, tape or string will also keep, dirt out, but they aren't as sure against spills.

A piece of tape over each hole in the top of the can works fine as long as the top and sides of the can are free of oil so the tape can adhere.

There are also tricks to opening a can of oil and pouring it down your car's oil-fill hole.

Probably the most common method is to place a screwdriver point on top of the can and whack it with the heel of your hand. Fine, except some oil cans are not made of metal - the tops are metal, but the cylindrical part is specially-treated paper. So instead of getting a hole when you hit the screwdriver, the can compresses and oil squirts out all over you. What I usually do with these paper-type oil cans is use a Phillips screwdriver (its pointed tip pierces easier) and hit it with a rubber mallet.

The secret is speed, to get the screwdriver through the top before the container can compress. With your hand you tend to hold back. But with the rubber mallet you don't. And the rubber mallet doesn't damage the screwdriver handle.

When you use a screwdriver to open a can of oil, punch a hole on either side of the top of the can, one for an air hole, and the other for the pour hole.

Just about any type of can opener works well, too. Many mechanics carry one of the universal can-bottle openers in their tool box. That's the job with a sharp tip on one end for opening cans, and a little hook and lip deal on the other end for removing bottle caps. Unfortunately, with the increasing use of twist-off bottle caps and easy-open cans by the beer and soft-drink companies, these openers are getting harder to find.

Another way is to spend a couple of bucks on one of those spouts you see pump jockeys use in service stations. They're available at auto-parts stores. With these you just stick the spout down in the can (it automatically pierces the can as you push it in), then stick the spout down the oil-fill hole and let the oil run out.

If you are going to pour directly from the can (as when you use a can opener or screwdriver), invest a buck in a small funnel. Otherwise you'll spill oil all over the engine trying to pour down the oil-fill hole.