Sometimes saying thank you is outside of your comfort zone. Do it anyway!
I know that for me (who, according to personality tests is both an introvert and an extrovert… explain that one), every day of going outside of your comfort zone would end up personally exhausting instead of invigorating, so this day’s challenge only needs to be done once, or as applicable. Still thank someone every day, but if you don’t make it out of your comfort zone, that’s okay. Or if thanking ANYONE is out of your comfort zone, today is just an extra day to practice that skill (and if it’s out of your comfort zone, you’re making it too hard. work on making it easy by slipping int into conversation. It’ll get easier) before adding a new one.