What does 'TARDIS' stand for on Doctor Who? And how come its interior is always changing while its exterior remains the same? Never fear; we have the answers.

The time-traveling box in "Doctor Who," called the TARDIS ... then use it to build a blue telephone booth. Time travel is tricky and full of risky paradoxes — like the grandfather paradox ...

It's hard to imagine Back to the Future without the DeLorean, but Doc Brown's time machine was almost something completely different.