"We often read that the fastest thing in the Universe is the speed of light. Why do we have this limitation and can anything possibly be faster?" asks Ali Alshareef from Qatif from Saudia Arabia. The team grapples with Einstein's Special Theory of Relativity, with help from cosmologist Andrew Pontzen and a British train, travelling somewhat slower than the speed of light. Plus physicist and presenter Jim Al-Khalili describes how he nearly lost his boxer shorts in a daring bet concerning the speed of subatomic particles.

The Cosmic Egg

"How do we measure the age of the Universe?" asks Simon Whitehead. A hundred years ago this wouldn't even have been considered a valid question, because we didn't think the Universe had a beginning at all. Even Einstein thought that space was eternal and unchanging.

This is the tale of how we discovered that the Universe had a beginning, and why calculating its age has been one of the greatest challenges in modern astronomy. We also uncover the mysterious dark energy that pervades the cosmos and discover why it's been putting a scientific spanner in the works.

Helping to unravel today's question are physicists Andrew Pontzen, Jo Dunkley and Jim Al-Khalili.

Picture: Star sun supernova galaxy gold, Credit: Eastern Lightcraft/Getty Images

Producer: Michelle Martin