When you have a best friend, developing a secret handshake is one the key ways to cement that friendship. Pulling off the handshake is sign you and your pal are in sync, that you get one another on a subliminal level.
Of course, your best friend doesn't have to be of the same species.
Case in point: this human and her black cat friend. Their handshake (pawshake?) is pretty darn cute, and a clear sign that they're spending plenty of close time together. A series of hand-to-paw taps, including high fives and fist bumps, ends with the cat hopping through a hoop the human makes with her arms. It's maximum adorableness and maximum friendship.