The Hanging Key is the fourteenth puzzle in Virtual Villagers 3.
In order to solve this puzzle mix 1 part Cactus + 1 part Rose + 1 part of Lotus = Light and Springy potion.
By now you probably noticed a bright pink square that is on the top right side of the screen, over the Orchard. That is the item that you need to pick up.
Have a villager drink the potion and drag them over to the pink square. The villager will jump up, grab the key and take it to the door.
As you fly the villager over the pink key you should see the words, "Trying to reach the key" under the 'action' section of your villager's description. That is your queue to drop the villager.