Sneak out of the house directly after evening 'prep,' and meet me in the playground, and I'll show you what it is." At the time appointed Acton ran down the path, and found Diggory waiting for him by the shed.
"Look," said the latter, "I've cut a little tiny slit with my knife in each door-post, about three feet from the ground, and I'm going to stretch this piece of black cotton between them.

No one will see it, and if they go through the door, the thread will simply draw out of one of the slits without their noticing it, and we shall see that it's been disturbed.

Jack Vance says that when he's been out shooting with his guv'nor he's seen the keeper put them across the paths in a wood to find out if poachers have been up them.

Now unlock the door, and let's go inside." In front of the bench, where the ground had been much trodden, there was a great deal of loose dust.

Diggory went down on his hands and knees, and producing an old clothes-brush from his pocket, swept about a square yard of the ground until the dust lay in a perfectly smooth surface.
"There," he said, rising to his feet again; "we'll do this the last thing every night, and any morning if we find the cotton gone we must look here for footprints, and then we ought to be able to tell if it's a man or a boy." "Don't you think we ought to tell Blake about that man you saw ?" asked Acton, as they walked back to the schoolroom.
"Well, I don't see how we can," answered Diggory.