Update: I was unfortunately not aware of Shamus Young's severe criticism of Fallout 3 available here to link in the original piece and I regret that. It dovetails rather nicely with what I've written and it's much better executed than my piece. I strongly recommend anyone...
When you get stuck with what to do call on Mechanic Pig and it will instantly build the best possible vehicle for a specific puzzle. You'll still need to ferry the pig to the end of the stage while reaching the assigned goals.
You can score three Mechanic Pigs by liking Bad Piggies on Facebook. When those run out, buy more via in-app purchase:
10 Mechanic Pigs: $1.99
35 Mechanic Pigs: $5.99
65 Mechanic Pigs: $9.99