Eren's eyes bulge when he sees it, having totally taken his coworker-turned-fuckbuddy for a square up until this very moment. "Stop staring; do you want me to get soft or fuck you?" Levi sounds really irritated, and Eren has to stop himself from laughing.
"Sorry, I just..." Eren doesn't know what to say exactly, so he asks the first thing that comes to mind, "can I put it in my mouth?"