Nagisa's momotalks come across like someone trying way too hard in front of sensei. Trying to see mature, trying to seem well put together, trying to act proper, but there are always gaps in her behavior which sensei instantly sees through

She orders a fancy new tea to try with sensei
The company sent her dried kelp by mistake
She makes kelp tea while pretending it's fancy tea because she doesn't want to admit her mistake
After they have their afternoon tea, sensei casually says he always knew it was kelp
She decides to try erasing his memory since he keeps seeing her embarrassing side every time they meet