Wait a sec, when you have a villager clean the beach after a storm, I believe that s/he gains construction skills, right?

Also, I currently have two master builders left (67 & 75) and only the 12th puzzle left. So, since I don't have a lot of time left to find the last one, I want to make it count; though I don't want anyone to just hand over the solution. That being said, do I have to do something with my masters or do they have to do something themselves?
Thanks,
-spaceman