White plays 16 to make an eye, and hopes that there will be room for a second eye on one side of 16 or the other. Actually, this group is still not quite alive, as we will see a little later in the game. But Black cannot find a good way to attack, and decides to play 17 to put the white cutting stone into atari. This is a good idea. Can the white stone get away? No, because it is caught in a ladder. If White tries to get away, Black will be able to play atari again and again until the white stones are trapped against the side of the board and captured. White sees this and wisely decides not to try to save this stone. Instead, White plays 18 to try to grab some territory in the last corner. Since White invaded the top right corner and made a group which is (almost) alive, Black decides to try the same trick with 19. This is not a good idea. This stone is on a 2-2 point, and is far too close to the corner to make two eyes. White plays 20 to surround this stone, Black tries to get away with 21, and White blocks the way with 22. The two Black stones are now trapped, and their only hope lies in some kind of counter-attack on the white stones that are surrounding them. So Black plays 23 to cut the white stones into two pieces in the hopes of capturing one or the other. This is a good type of strategy which often works.