Cut out the hard center core, using a knife (use a petty knife if available). For an iceberg lettuce, use a ceramic knife and take out the central core with your hand.
Run tap water (not so briskly) into the gap between the leaves. The water accumulates in the gaps and the leaves come off naturally because of the weight of the water in the gaps.
With this method, you can remove whole leaves without damaging. After several leaves are removed, score around the center core to remove other leaves.
Story Behind this Recipe
I consulted a cookery book a long time ago to make just several stuffed cabbage rolls. According to it, I had to boil a whole cabbage in a pot. It was too much and I just wanted to use several leaves. It is difficult to remove cabbage leaves without tearing before boiling. Since I thought of this method, I have always been successful.
Use a small knife to cut out the hard center core. The leftover cabbage will freshen up because of the water! This is another good point of this method. If a metal blade touches the cut surface of a lettuce, it will discolour. Use a ceramic knife or your hand to cut the leaves from the center core.