For many dishes, you don't have to leave the apple intact when you core it, such as for making pies, tarts, applesauce or slicing apples to eat. But if you want to bake whole apples or make apple rings, you need to be able to remove the core while keeping the rest of the apple whole. An apple corer makes this easy, but you can do the same thing with a paring knife.
Rinse the apple under running water, then pat it dry with a paper towel.
Place the apple on a cutting board, right side up with the stem pointing up.
Insert the paring knife straight down into the apple, 1/4 inch away from the left side of the stem. Make sure the knife reaches the bottom of the apple, and pokes through so you can see the blade. Repeat this at the top of the stem, on the right side and on the bottom, making a square around the stem.
Insert the knife into the first incision again, and slowly saw straight through to the second incision. Then move onto the third, then the fourth, then back to the first incision. Make sure the knife's point has reached all the way through the apple to the bottom. This carves an entire circle around the stem.
Set the apple down on its side on the cutting board. Use your thumb to push the core of the apple through. Do this carefully and slowly. If it doesn't come out easily, use the knife to cut any parts still attached. Slowly push out the core until it is entirely out of the apple.