What trick was played on Petro and Crater?

It’s the 22nd Century. A tough, pioneering people mine the moon for Helium-3 to produce energy for a desperate, war-torn Earth. Sixteen-year old Crater Trueblood loves his job as a Helium-3 miner. But when he finds courage he didn’t know he had and saves a fellow miner, his life changes forever. Impressed by his heroism, the owner of the mine orders Crater to undertake a dangerous mission. Crater doubts himself, but he has no choice. He must go.

With the help of Maria, the mine owner’s frustrating but gorgeous granddaughter, and his gillie—a sentient and sometimes insubordinate clump of slime mold cells—Crater must fight both human and subhuman enemies. He’ll battle his way across a thousand miles of deadly but magnificent lunar terrain before vaulting into the far reaches of space, there to recover an astonishing object that could mean the difference between life and death for every inhabitant on the moon.

what does the teacher mean