Geographically, it is a location not far from the shores of the continent. The left cliff was voiced by Tom Kenny and the right cliff was voiced by Jeff Bennett.
The Zoombinis have to cross a deep gorge using one of the two bridges. Unfortunately, the cliffs are allergic to certain types of Zoombini features (i.e. hair, eyes, noses, or feet). You need to test which cliff is allergic and to which features and figure out which Zoombinis should cross which bridge. Every time when a Zoombini crosses a bridge but the cliff is allergic to it, the cliff will sneeze and one of the pegs that are keeping the bridges stable will fall off. If all pegs are lost, the bridge will collapse and trap the rest of the Zoombinis.
In-game Help Text
Not so easy
The allergic cliffs play by certain rules. What one cliff accepts, the other is allergic to. Look for one feature, such as a red nose, that does not cause an allergic reaction.
Oh, so hard
The allergic cliffs play by certain rules. What one cliff accepts, the other is allergic to. Look for two different types of the same attribute, such as red and green noses, that does not cause an allergic reaction.
The allergic cliffs play by certain rules. What one cliff accepts, the other is allergic to. Look for two different features, such as red noses and sunglasses, that do not cause an allergic reaction.
Very, very hard
The allergic cliffs play by certain rules. What one cliff accepts, the other is allergic to. Look for three different features, such as red noses, sunglasses, and spring feet, that do not cause an allergic reaction.
Detailed Mechanics and Rules
Misc Notes: Either bridge can be the one with the positive rule.
Strategy for Solving the Puzzle
To solve the puzzle, the player has to figure out which cliff is the one that is allergic because the other cliff will simply sneeze when the Zoombini belongs to the other bridge, despite not being allergic. After finding the allergic cliff, look at the Zoombinis at the non-allergic cliff's side and figure out the features the allergic cliff is allergic to by process of elimination (i.e. if the non-allergic cliff accepts sunglasses, the allergic cliff is probably allergic to those unless if a Zoombini with sunglasses is accepted by the allergic cliff, in which you can deduce that the allergic cliff is NOT allergic to sunglasses.)
The Cliffs were formed by a small river. After many, many years of erosion, the cliffs were created. In the process of doing so, the faces were formed. An unknown figure built the bridges that lead across the bridges. Zoombinis traveled across these bridges in their journey. It was discovered then that the Cliffs were allergic to certain Zoombini features, and built up more more complicated allergies as time went on.