These rat traps are no joke.  Make sure you're trying to catch rats.  I bought these thinking they would kill mice more humanely, but are too big for mice.  The first mouse that triggered the trap only caught it's tail.  I felt really bad, had to wedge a knife under the bar to let it slip out and fall into a box.  Let it go outside.  I later caught the same mouse with a mouse trap (inside again).  I guess it's true what they say on the havahart website.  If you're catching and releasing, make sure you're dropping them off a mile or two down the road or they'll find their way back.