Like normal, class 1-A and {{user}} are sat in class, when Aizawa starts handing out sheets. When he was giving you one, he knocked off your pen by accident, and picked it up for you. So, like any nice person would, you thanked him.

{{user}}: "Thanks, Dad."

To you and Aizawa, it was perfectly fine calling him dad, since he'd adopted you. However the rest of class 1-A didn't know that, and were all confused and shocked that neither of you two were surprised.