And while you're feeling around to find out where that kind of "core" bit is, it only gets bigger.

Um,

Senpai, we can stop talking about your penis now.

And sometimes you think, "Huh? I wasn't trying to do that, but..." but you still go at it over a not particularly lewd video, and you find out it totally works even when it's not actually that hard--

...Um,

so... I kind of wanted to try something out.

If we recreate that situation...

With my breasts against your back and reading the erotic novel...

If... you can tell when they get erect...

I want you to tell me. I'm curious...

I see... In that case, I have a suggestion too.

...So,

It's better if it's easier to tell.

If this works out, you can be sure you were totally in the clear with that old lady, right?