A ticking clock is good for adding suspense. What device does Lessing use to create a sort of "ticking clock" that increases our anxiety?
Lessing creates tension and suspense by using a sort of ticking clock in the climax of this story. Jerry knows that he counted to one hundred and sixty while the older boys swam through the tunnel, so he knows that he has to be able to hold his breath for the same amount of time in order to accomplish the same feat. As he swims through the tunnel, he keeps count in his head, "Fifty, fifty-one, fifty-two . . . . [...]. Seventy-one, seventy-two . . . ." He soon reaches one hundred, and "His lungs were beginning to hurt." Then, he reaches one hundred fifteen. A few moments later, he counts one hundred fifteen again, but he knows he already counted to that. "His head was swelling, his lungs cracking. A hundred and fifteen, a hundred and fifteen pounded through his head, and he feebly clutched at rocks in the dark [...]. He felt he was dying." When Jerry gets stuck on one hundred fifteen, he loses track of how long he's truly been underwater, and we have no idea how much further he can make it (nor does he know). This is the moment of the most tension in the story because the clock is still ticking, so to speak, but we have no idea how much time has passed. This is, literally, life or death for Jerry, and so the tension and anxiety are terrible.
There are two ways Lessing creates a "ticking-clock" effect in the story. The first is the realization by Jerry that he must learn to hold his breath at least two minutes. Each day he counts the number of second he can stay underwater. "Fifty-one, fifty-two...His chest was hurting" The counting intensifies the feeling of time passing by. In addition, Jerry is up against a time deadline because he knows he and his mother will leave the seashore at some time and he will not be back until the next year. Therefore, he is in a hurry to learn to hold his breath so he can accomplish his task before he and his mother return home. Both of these techniques suggest time passing quickly for Jerry. It's the time he has left of childhood before he begins growing into a young adult.