Our apartment complex has a rat problem...

They aren't in the house, but run around along the walls outside and it drives me nuts when I'm trying to sleep. Yesterday, I went down to the store and got some nice big rat traps, the snap kind, and loaded them up and set them in the back patio. Tonight, I heard that trademark SNAP! and went to check...sure enough I had knocked one of them off. Satisfied, I shut the door and went back to SRP. I had gotten him right on the neck and he wasn't moving. About ten minutes later, I heard a commotion on the patio and went to investigate....no more rat!!! Somehow this monster had gotten himself out of the trap! How the heck did he get out? These things would surely break my finger if I had it in the way when it went off, but somehow this rat managed to get his full head out of the trap.

any ideas?