I'm sure Mana was expecting Nakano to act really embarrassed from seeing that. Too bad though, she's built up a strong resistance to those kinds of thoughts.
Mana already knows about her seeing her own imaginations based on words, and seeing that reaction to those 'words' kinda proves the girl have had thoughts of that nature a fair bit already..
Kinda looked like someone realizing a whole new way to enjoy their toys tbh. =D