Smile

A smile was a facial expression that involved the turning up of one's lips, sometimes showing teeth, primarily serving as a sign of happiness. The Twelfth Doctor noted that smiles had a psychological effect on one's mood state. (TV: Smile)

The Ninth Doctor suggested that Blon Fel-Fotch Passameer-Day Slitheen would spare a potential victim every now and then "because she smiled" when he rejected her sparing of Cathy Salt as evidence that she had changed. (TV: Boom Town)

Shortly following his regeneration, the Tenth Doctor made Rose Tyler smile with a quip that, being familiar with her mother, nut loaf would be a more appropriate choice of Christmas dinner for the elder Tyler to prepare. (TV: Born Again)

Overseeing the operation of a hologram of Joseph Serf, Plark instructed his fellow Skullions to repeatedly make him smile as he conversed with Sarah Jane Smith. However, they made the mistake of giving him a "sexy smile", which Plark ordered amended to "thoughtful smile". (TV: The Man Who Never Was)

When recalling the events that had led to them saving Gallifrey at the end of the Last Great Time War, the Doctor noted that their otherwise morose warrior incarnation always smiled whenever he laid eyes on Clara Oswald. (PROSE: The Day of the Doctor) When the Twelfth Doctor later lost his memory of Clara, he requested that she smile for him one last time before he forgot her. (TV: Hell Bent)

The Emojibots and the Vardy, programmed to make an upcoming human colony happy, registered grief as the enemy of happiness, responding with lethal force when they detected it. As a result, the Twelfth Doctor and Bill Potts were forced to smile to avoid lethal countermeasures until they worked out the truth of the robots. (TV: Smile)

The Gendar, although humanoid in appearance, had a somewhat different body language from humans'. Among other things, Maxie Masters, a Gendar living undercover as a human in the 2010s, had some trouble learning how to smile in a way that humans found acceptable; her best attempts, even four years after coming to Earth, were "irregular", "all teeth and no expression". (AUDIO: Broken Bonds)