Each fact is suggestive in itself. Together they have a cumulative force.

We must fall back on the old axiom that when all other contingencies fail, whatever remains, however improbable, must be the truth. Here all other contingencies have failed.

“The string is exceedingly interesting,” he remarked, holding it up to the light and sniffing it. “What do you make of this string Lestrade?”
“It has been tarred.”
“Precisely, it is a piece of tarred twine. You have also, no doubt, remarked that Miss Cushing has cut the cord with a scissors, as can be seen by the double fray on each side. This is of importance.”
“I cannot see the importance,” said Lestrade.
“The importance lies in the fact that the knot is left intact, and that this knot is of a peculiar character.”
“It is very neatly tied. I had already made a note to that effect,” said Lestrade complacently.

“The principal difficulty in your case,” remarked Holmes, “lay in the fact of there being too much evidence.”

“Or towards it?”
“No, no, my dear Watson. The more deeply sunk impression is, of course, the hind wheel, upon which the weight rests. You perceive several places where it has passed across and obliterated the more shallow mark of the front one. It was undoubtedly heading away from the school.”

Singularity is almost invariably a clue.

I knew … that his mind, like my own, was busy in endeavouring to frame some scheme into which all these strange and apparently disconnected episodes could be fitted

It is of the highest importance in the art of detection to be able to recognise out of a number of facts which are incidental and which vital.

Well, of course, if the case were not an odd one we should not have been driven to ask you for an explanation.