You will need to have solved Puzzle 3. Collect the following herbs for a potion:
Cactus, Rose, Lotus. Make the potion and have a villager other than a nursing 
mother drink it. That villager should feel "light and springy". Drop them near
the pink diamond (the hanging object) just above the rose plant. Their action 
should read "Trying to reach the key". They will jump up and collect the key.