Your mother taught you to say “thank you” whenever anyone gave you anything, and just because you’re all grown up now doesn’t mean the rules have changed. Saying “thank you” shows the giver that you appreciate the gift or favor, and helps to better cement your relationship than unrequited gift giving. And while a child can get away with nothing more than yelling “Thanks!” over their shoulder as they run off to play, adults sometimes have to send a card.