Likely cause: Sweaty shoes.
What to do: The damp, dark inside of your sneaks is a breeding ground for smelly bacteria, so wear absorbent cotton socks with shoes made from breathable materials, like canvas and leather. Wash your feet daily with antibacterial soap, and throw a tea party for your toes a few times a week (pour a pot of green tea made with several tea bags into a basin; let cool until warm, then soak for 30 minutes). "The tea contains powerful polyphenols that knock out organisms that make your feet smell," says Dr. Roizen. If this doesn't help, talk to your doc because the odor could signal a fungal infection. Athlete's foot, a fungus that makes skin itch and burn, is treatable with OTC creams, while toenail fungus, which often starts with nail discoloration, requires prescription antifungal pills or ointment.