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This two-part special presented by Hannah Fry shows that when it comes to the universe, size really does matter. Hannah takes the audience into a thought experiment where the size of everything can be changed to reveal why things are the size they are.

Having discovered in the first episode that making things bigger ends in disaster, in episode two, Hannah is going the other way by asking whether everything could, in fact, be smaller. But going smaller turns out not to be much safer...

First, we shrink the Earth to half its size - it starts well with lower gravity enabling us to do incredible acrobatics, but things gradually turn nasty as everyone gets altitude sickness - even at sea level. Then we visit Professor Daniel Lathrop's incredible laboratory, where he has built a model Earth that allows us to investigate the other effects of shrinking the planet to half size. The results aren't good - with a weaker magnetic field we would lose our atmosphere and eventually become a barren, lifeless rock like Mars.

In our next thought experiment, we shrink people to find out what life is like if you are just 5mm tall. We find out why small creatures have superpowers that seem to defy the laws of physics, meet Jyoti Amge, the world's smallest woman, and with the help of Dr Diana Van Heemst and thousands of baseball players reveal why short people have longer lives.

Lastly, the Sun gets as small as a sun can be. We visit the fusion reactor at the Joint European Torus to find out why stars have to be a minimum size or fusion won't happen. And if our Sun were that small? Plants would turn from green to black, and Earth would probably resemble a giant, frozen eyeball.

Which all goes to show that size really does matter.