If you spend all day admiring yourself in reflective surfaces — mirrors, pools of water, the backs of spoons — people may think you are conceited, or vain.
Vain is from Latin vanus, "empty," and in English it originally meant "lacking value or effect, futile," like your vain attempt to find a pencil in your messy, overstuffed backpack. Likewise, the phrase "in vain" means "without success." Vain also means "conceited, too proud of oneself." Carly Simon's line "You're so vain, you probably think this song is about you" is an excellent illustration of this use.