Who was the 11 year-old that spoke of Fourth Dimension figures with 120 sides (hecatonicosihedrigons) and 600 sides (hexacosihedrigons)?

"Actually, it is possible to take any figure of the third dimension and use it to construct four-dimensional figures," says William. "These figures of the fourth dimension are called polyhedrigons. In this way it is possible to construct figures in the fourth dimension with 120 sides called hecatonicosihedrigons..."

Whispers can be heard throughout the hall; some hands go up, and Professor Hauptman rises from his seat. "Young Sidis," he says--but William does not hear Professor Hauptman.

"It is also possible to create figures with 600 sides called hexacosihedrigons, and you can figure out parallelopipdons that are..."

"Sidis!" says Professor Hauptman, louder.

William turns from the board.

"Mr. Sidis, excuse me, but I must interrupt you. Where did you get them?"

"I don't know what you mean, sir," says William.

"Where did they come from ... these hedigrons?"

"From the fourth dimension," says William.