Mable had sort if been expecting Edward's rejection of the shell, but it still made her frown.  But then the boy continued, and his words were almost like a challenge, and it made Mable laugh.

"You're so smart, Edward, that's a good point!" she said in between her giggles, taking the shell back and replacing it with the green sea glass.  "I'll treasure this shell forever now, so you have to do the same with yours.  If you lose it, I'll be really sad and cry, so don't, okay?"

Her words were serious, but Mable said them with a smile.  She looked down at the shell again and placed it on the desk, away from the rest of the collection so she would be able to see it at a glance.

Quietly, she wondered Edward wanting her green shell was just out of friendship, or because he thought her eyes were pretty too, and she was tempted to ask, but just the thought of doing so made her cheeks feel hot.  It was probably jus because they were friends like he said anyway.  And she knew her eyes were pretty!  She didn't need Edward to think so, or tell her that.  Even if she'd really like to hear him say it.