Good evening Netizena…
The vet explained to the children that what appeared to be a display of anger in the cat was actually a behavior called flehmen.
“One of the behavioural components of male sexual display in all hoofed stock except the pig is the 'olfactory reflex' known as flehmen. In this reflex, [the] animal fully extends the head and neck, contracts the nares and raises the upper lip while taking shallow respiration.” — From an article by S. Gul in the Journal of Animal and Plant Sciences, September 30, 2012
“Flehmen” comes to English by way of German, in which language the word applies to animals and means “to curl the upper lip.” The German source of the English word is a verb, and the English word is also used—albeit rarely—as a verb, as in “the horse flehmened.” More often, though, the verbal form is actually a gerund: “the horse's flehmening.” “Flehmen” is sometimes capitalized in English, as nouns are in German, but more often it is lowercase.