Nina investigates why people's backs are so bumpy, with the help of Felix, her touch neuron.
Experimenters Cara, Kai and Rianna visit Nina in her lab and find out that backs feel bumpy because the backbone - or spine - is made up of lots of little hard bones. These protect something called the spinal cord which the brain uses to talk to the rest of the body.
They go to an exercise class and do lots of bendy, stretchy moves. Nina explains that people are able to bend and stretch in lots of different, useful ways because the spine is made up of lots of little bones, not one big, long, hard bone.
Back in the lab, they do a smashing experiment with eggs and discover that there are little soft discs between the bones in the spine. These act as little cushions, stopping the bones from bashing together when people move.