Sirius Black high-fiving Lily Evans every time she raises her hand in class.
Sirius asking James to high five her when he is took sick to come to class.
Remus Lupin doing it with a sigh and an immediate apology when neither Sirius or James is available.
Peter being overly enthusiastic about being asked to do it, and slapping Lily in the face by accident.
Wizards can make fun of muggles all they want but the joke’s on them because phones can do in two seconds what they use owls and high-level magic to do, AND we have tanks. Try to avada kedavra a tank you stick-wiggling nerds