Took him roughly 10 min to get to the point where there was only a tail sticking out of his mouth.
As far as how I prekilled, it's not for the faint of heart but it works like a charm. An old friend showed me this back in the days when I was big into reptiles...
To put it simple: Break the mouse's neck.
To put it explicitly: Place a pencil/screwdriver/stick between the mouses head and body(on it's neck, I guess). Pin it down on a flat surface and yank it's tail. Dead mouse.
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