Symbolism, Imagery, Allegory
A few years before the events of The Wild Children, Alex is given a very special gift from his tricky Uncle Dmitri: a stone that he claims will "turn to gold" if Alex can manage to "hold it" and "not think of camels" (1.6).
This is a prank, of course. Good old Uncle Dmitri knows that his anti-camel warning will make it impossible for Alex to think of anything but camels while he holds the stone, driving the poor kids insane while still keeping him convinced that magic is real. It's like if we told you to read the rest of this section without thinking of chocolate ice cream. Just try it. We dare you.
A similar thing happens while Alex is starving on the road to Moscow. This time, however, it's not a stone that he's obsessing over; it's a steaming bowl of soup. Regardless, Alex finds the same exact thing happening to him here, with his attempts to "not think of soup" only making his fantasies of it "warming [his] throat" (3.1) all the more vivid. At least he's not thinking of camels anymore, right?
Although Alex probably doesn't see these two things as connected, their similarities are pretty obvious from where we're sitting. To put it in the simplest possible terms, it's impossible to look away from the thing that's staring you in the face.