He leaned forward and twanged the strings of his 'cello.

They were not in tune.
"Will you lend me your tuning-fork ?" he said to Aubrey.
But Aubrey had expected this.
"Sorry," he said.

"I don't possess one, just now.

I gave away mine last week.

You can tune your 'cello by the organ." "I don't know how to tune a 'cello," said Ronnie.
"Let me show you," suggested Aubrey, with the utmost friendliness.
He walked over to the organ, drew out the 'cello stop, sounded a note, then came back humming it.
Then he took up the Infant and carefully tuned the four strings, talking easily meanwhile.
"You see?
You screw up the pegs--so.