In the middle ages lions really were kept in the Tower of London. By Victoria's reign in 1837 all the lions had been moved to safer accommodation in Regents Park. In 1860 April Fool's day fell on a Sunday, so a prankster had an idea to invite 'all and sundry' to the Tower of London to see the annual washing of the white lions.
The invitation said: Admit the bearer and friend to view the annual ceremony of 'Washing the white lions' on Sunday, April 1st, 1860. Thousands of people turned up and waited, until one-by-one, it dawned on them that they had been hoaxed.