Final answer:
Marcus is performing a surprising feat of putting a billiard ball into his mouth, which contrasts with the humble past experiences shared with McTeague.
Step-by-step explanation:
The passage provided depicts a complicated relationship between two friends, Marcus and McTeague, and their shared affections for the same girl, Trina. Interspersed with moments of tension and camaraderie, their interactions oscillate between friendly jests and underlying conflict.
In one instance, Marcus showcases his ability to perform a seemingly impossible feat by putting an entire billiard ball into his mouth, which amuses McTeague. In the opening line, a significant change in McTeague's circumstances is implied, suggesting that at present, Marcus is doing something highly unexpected or impressive, far removed from the humble beginnings of sharing a mattress with his father.