“You look as if you had had a hard time catching the creature, whatever it is,” said nurse. “You’d better leave it out-of-doors now, and clean yourself, and come and eat your luncheon.”

“Oh, please, nurse, let’s see it now!” said all the children; and nurse, who wanted to see it herself, agreed.

“You can’t see it,” said the Professor; “it’s invisible! You can’t see it till it disappears!”

“Oh dear,” said Pip, “I just ache to know about it.”

“Well,” said the Professor, “light mamma’s wax-taper.”

“I don’t see what good lighting a taper will do, if the creature’s invisible,” said Bob.

The Professor set his burden down on the table. It was a saucer filled with water, and in the water stood a tumbler upside down. There was nothing to be seen in the tumbler.

The Professor struck an attitude.

“What I have in this tumbler, nurse and children, was obtained with great difficulty. I’ve been about it ever since lesson-time.”

“Where did you find it?” says Pip.