As soon as he walks into the room, flashing you a quick grin, your cheeks take on a light shade of red. You feel the crimson washing over your face, but there's nothing you can do — you're blushing.
Turns out, this cheek-coloring reaction is a universal human response to social attention. Everyone does it, some more than others. Common blushing triggers include meeting someone important, receiving a compliment, and when experiencing a strong emotion in a social situation. Apparently guys don't color their cheeks red as often as women.
Blush biology works like this: Veins in the face dilate, causing more blood to flow into your cheeks, thus the rosy color. But scientists are stumped as to why it happens or what function it serves. That is, besides deflating your ego.