I just found an adorable trope thanks to a discord username..
(C.[A,B.C]. = Character.[A,B,C].)
Character A and B have been friends since forever, longer even. C.A. blatantly flirts with C.B. and C.B. remains clueless and C.C. is tired of their shenanigans.
C.A. holds up their fingers, eyes widening in pleasure. "You know, there are only 21 letters in the alphabet," they say smugly.
C.B. looks up, shaken. "What? No, you're wrong. There are twenty-"
"Oh, yeah. You're right. I forgot U, R, A, Q, T."
C.B. pulls a blank face. "How did you forget them?"
C.A. falls to the ground in despair, "Aaaaa, you'll never get it C.B.!! C.C. tell them!"
C.C. glances up from reading their book, wrinkling their nose at C.A., "No."