There are two ways to increase a builder's skill in Virtual Villagers: The Secret City. The first is to keep putting villagers on the huts that they constructed, making sure that their action says, "Fixing hut" (and not "Examining hut"). If they are successful, they will gain building skill. The second way is to make a specific potion, which will cause the villagers to exercise. If they successfully complete the exercise regimen, they will gain building skill.
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