Why does catnip make my cat go crazy?
The organic compound in catnip, nepetalactone, is what gives cats these bizarre reactions. Once the nepetalactone enters the cat’s nose, it binds to the olfactory cells which send their signal to the brain.

Nobody quite understands why it causes strange effects in cats, but some scientists believe that the brain interprets them as cat pheromones which causes their behaviour to go haywire. Aside from this, it acts as a natural mood enhancer, which may explain why cats like catnip so much!
Oh, that's really interesting! Are there any equivalents to catnip for dogs?