General hints: 1) No one ever answers grey doors. Ever. I wish someone did. 2) Magic cans - they can be used whenever there are a lot of birds. 3) Bombs are sometimes useful, especially on fire hydrant. 4) Jump on enemies; don't can-them. You get free stuff that way. 5) Always recycle cans (money is good for buying things); I recommend buying up until you hit a multiple of $5 with a few cans left for emergencies.
