What's stopping us from building useful quantum computers? And how long until we'll have them?

Why it's hard to build quantum computers and what can be done about it.

An untapped resource could provide the magic needed for quantum computation — and perhaps even open the door to time travel.

Some neat number tricks using numbers read backwards.

Eleven makes multiplication easy.

A quick look at an ancient problem.

The impossible becomes possible when you move into the third dimension.

A mathematical "paradox" explains why more roads don't necessarily mean fewer traffic jams.