How does Karel know when not to continue moving in a challenge?

Karel knows not to continue moving when it encounters a wall because the programming rules dictate that Karel must stop when it comes up against an obstacle that it cannot pass through. This is a fundamental aspect of Karel's programming, emphasizing the concept of boundaries in a grid-like environment where Karel operates. The wall acts as a physical barrier preventing further movement in that direction, ensuring that Karel adheres to the rules of its environment.

In contrast, losing track of the balls or having a maximum number of moves would not directly influence Karel's movement in the same way; rather, those scenarios could lead to a change in behavior or tasks that Karel might undertake. Being out of energy is not a specified condition in basic Karel programming challenges and typically doesn't apply to the fundamental movements Karel can perform.