Glass 1 and 5 have their outlets blocked from the start.
Glass 2 is blocked at the end of its tube.
Glass 3, 4, and 7 have obstructions in the middle of their tubes.
Glass 6, however, is simply not connected to anything else.
Result: the water has no free passage to completely fill a glass.
The correct (and most frustrating) answer:
No glass will be refilled.
This is often the response that elicits a "Sure!" followed by mild irritation. Yet, once the explanation is provided, everything seems obvious. This demonstrates how our eyes can be fooled by a well-thought-out visual presentation.
Why this type of puzzle is so effective:
These puzzles appeal to two very human reflexes:
