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Look for one with a rough surface. A smooth rock won't create enough friction to puncture the lid of the can. Positioning it upside down will allow you to break the seal, which is located on the top of the can. Use a scrubbing motion to create friction between the rock and the can. Keep going until you see moisture appear on the rock or on the lid of the can. Turn the can over to check it every so often. You want to stop as soon as you see moisture. This means the lid is thin enough to break through. Don't rub so hard that you rub right through the can lid. Your food will spill all over the rock. The seal should be thin enough to allow you to easily slide the blade into the can near the edge of the lid. Push up with the knife to gently pry off the lid. Finish pulling off the lid, then discard it. If you don't have a pocket knife, try using a spoon, a butter knife or another tool. Or find a rock you can use to knock the lid of the can inward. This isn't ideal, since you may contaminate your food with small bits of rock or dirt. When you pull off the lid, cover your hand with your sleeve or a towel so you won't cut yourself.
Find a flat rock or a piece of concrete. Position the can upside down against the rock. Rub the can back and forth over the rock. Use a pocket knife to pry open the lid.