Nina investigates how we know the Earth is round - with the help of Luke, her sight neuron. Explorers Charlie, Katie and Zack visit Nina in her workshop and discover that even though the Earth looks flat it's actually curved. Next they release a really big balloon that goes way up into the sky to show that the Earth is round. Lastly they travel around Nina's big inflatable Earth and find out that if you go all the way round in the same direction, you come back to where you started from.