I'm not sure about the American versions of these but in Australia, such traps are a 'box' made of wire mesh (the rigid sort) with a door at one end that is hinged and has a vertical extension on it so that, when open, the extension is held in place (and thus the door is open) by a catch that releases the extension if either a pressure pad (that the animal treads on when inside) or the bait is disturbed. The door shuts and the animal is unharmed by the process.

There are various patterns and design wrinkles that prevent long-tailed animals from either getting their tails caught or using them to jam the doo open but I'm sure you've got the picture. Dpepending on the animal's regard for the subesquent experience you may only ever catch it once or it will become a regular visitor, a problem wildlife surveyors have to cope with. You must check a set trap every eight hours (at most) or the animal ethics people will have your guts for garters, and it's only fair out of respect to the animal, no matter how 'verminous'.

Cheers, Rowan