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