Then he tried again, but still the bolt would not move.

So now he tied his handkerchief--the one with the bacon-fat and marmalade on it--to the bolt, and Robert's handkerchief to that, in a reef knot, which cannot come undone however much you pull, and, indeed, gets tighter and tighter the more you pull it.

This must not be confused with a granny knot, which comes undone if you look at it.

And then he and Robert pulled, and the girls put their arms round their brothers and pulled too, and suddenly the bolt gave way with a rusty scrunch, and they all rolled together to the bottom of the stairs--all but the Phoenix, which had taken to its wings when the pulling began.
Nobody was hurt much, because the rolled-up carpet broke their fall; and now, indeed, the shoulders of the boys were used to some purpose, for the stone allowed them to heave it up.

They felt it give; dust fell freely on them.
'Now, then,' cried Robert, forgetting his head and his temper, 'push all together.