They may have a crude sense of humor.
Those same old puns and riddles might not cut it anymore. Third graders may tell dirty jokes, and they are quick to share with their friends. They may laugh at the misfortune of others. They have a unique tendency to giggle uncontrollably and let humor deter their focus for long stretches.
What you should do: Keep your cool and give your child reasonable limits, such as, “You can joke and laugh during lunch but not during class.”