Final answer:
An "Achilles heel" refers to a person's greatest weakness, originating from the Greek myth of Achilles whose heel was his only vulnerable spot. In the provided text, it suggests that the balance beam could be Khalil's weakness in an obstacle course.
Step-by-step explanation:
The reference to an "Achilles heel" in the sentences above alludes to a person's vulnerability or weakest spot, despite overall strength. Drawing from Greek mythology, this term originates from the story of Achilles, whose heel was his only vulnerable point where he could be wounded, as the rest of his body was invulnerable due to being dipped in the River Styx by his mother Thetis.
The use of the phrase indicates that while Khalil is confident in his ability to navigate the obstacle course, he is worried that the balance beam could pose a significant challenge and potentially lead to his downfall—much like Achilles' heel ultimately led to Achilles' demise.