A Unicorn is a mythical animal. But it's also the name now given to private start-up companies, mainly in the tech or internet sector which are valued at a billion dollars or more. They're extremely fast-growing and are often keener to increase customers rather than make profits at this stage. They rely on private investors to fund their growth and those investors give the companies their valuations. Through interviews with European unicorns including Blah, Blah Car, a ride-sharing service and Hello Fresh which delivers measured fresh ingredients and recipes to your door, Caroline Bayley asks how "real" the tech unicorns are and whether the billion dollar plus valuations are fuelling another tech bubble which could be in danger of bursting. Producer Anna Meisel.