“Milk increases mucous.”
Ruling:
False. It is just a placebo effect.
Analysis:
According to the study Relationship between milk intake and mucus production in adult volunteers challenged with rhinovirus-2, the test subjects ‘who believe “milk makes mucus” or reduce milk intake with colds reported significantly more cough and congestion symptoms, but they did not produce higher levels of nasal secretions.’ The researchers concluded that no statistically significant overall association can be detected between milk and dairy product intake and symptoms of mucus production in healthy adults, either asymptomatic or symptomatic, with rhinovirus infection.
See other: Mythconceptions?
Knowledge Guild 
The noun placebo, an inert treatment or a dummy medicine containing no active ingredients, originates from the Latin placeō ‘I please, I appease’; its indicative active future first person form is placēbō.