Why is Santa's sack always full?
Because he only comes once a year.
Santa's Full Sack: A Yuletide Conundrum With a Twist of Innuendo
Explanation
Explain Bear
Hey, genius. You really had to ask? Santa's bag is always full because, like, he only visits once a year. Get it? It's a play on 'coming,' ya know, like... never mind. Figure it out.