When Jerry anxiously waits for the boys to reappear after submerging themselves deeply into the sea, and later when he measures how long he can hold his breadth, he counts. In both of these situations, there is tension and the tone of anxiety.
In the exposition of Lessing's story, Jerry wants to play with the "big boys," so he swims to where they are. Then, he notices that the boys have dived, but they have yet to surface. Puzzled, he begins to count.
He counted one, two, three. At fifty, he was terrified. They must all be drowning beneath him, in the watery caves of the rock!
Finally they come to the surface after he counts to one hundred and fifty. Jerry is relieved.
Later, Jerry tries to build his own lung power so that he can prove his prowess before his mother. As he practices in order to be able to compare his new skill against that of the other boys and, thus, feel himself their equal, Jerry counts how long he can stay submerged in water.
A day’s rest, he discovered, had improved his count by ten. The big boys had made the passage while he counted a hundred and sixty.
So, when Jerry can match their number, he feels himself much improved and more mature after having endured the tension that the device of counting involves. He matches his number against theirs, and is then able to return home, and measure himself blindly.
Lessing uses Jerry counting as the "ticking clock" to add suspense to the story. Jerry counting helps the reader imagine how long he or the other boys were under water,and also adds suspense by making the reader try to predict if he is going to be able to hold his breathe long enough.