Crookshanks darted up first; he had evidently pressed his paw to the knot on the trunk, because Lupin, Pettigrew, and Ron clambered upward without any sound of savaging branches.
He dashed his head against the knotted trunk; and, lifting up his eyes, howled, not like a man, but like a savage beast getting goaded to death with knives and spears.
Sirius thought it would be -- er -- amusing, to tell Snape all he had to do was prod the knot on the tree trunk with a long stick, and he'd be able to get in after me.