Doctor Who Series 10: 12 Easter Eggs & References You Might've Missed In "Knock Knock'

Bill finally finds out that the Doctor is a Time Lord this episode - and in classic Bill style, she mocks him for it. "Do you wear robes and big hats?" she asks. "No, big collars mostly," he replies.

The Doctor is referring to the ginormous ceremonial high collars that have become synonymous with the Time Lords over the years. The collars first appeared in 1976's The Deadly Assasin, which saw the show's first proper trip to Gallifrey. Though the Time Lords had appeared several times before.

The Doctor also specifies that the Time Lords are his "species" here, which flies in the face of what we thought we knew about Gallifreyan life. It had always been assumed that Gallifreyan was the species but the Time Lords were the ruling class.