He picked up a bit of paper from the street and nailed it to the frame of the door with a knife. He then turned his back upon this place and walked to the opposite side of the street.
" When I shall throw some pebbles from my chamber, do not fail to rip open the jar from top to bottom with the knife you have and to come out; I shall be with you soon after."
Just as the frying-pan was nicely hissing Peter and Mr Beaver came in with the fish which Mr Beaver had already opened with his knife and cleaned out in the open air.
The expression " to go under the knife" means to have an operation, to have a surgery, to be operated on, where the doctor actually has to take a knife and cut you open.