If you have a vegetable peeler with a pointed end, you can use it to core a whole apple. The peeler should have a long vertical blade, and the plastic behind it shouldn’t be too thick. Place the peeler on the stem of the apple and towards the centre of the core. Push the peeler down, and then pull it back out. Turn the apple and do it again until you create a circle. It won’t be as clean as an apple corer, but it will do the trick. You can also just use a small paring knife. Cut straight down, remove, turn the apple and cut down again until you get all sides of the core.