Have you ever slipped into second-hand shoes and noticed a strange yellow bump on your toe? This problem is common and usually harmless. In most cases, it’s “a corn or callus—a thickened area of skin that forms as a protective response to repeated friction or pressure.” Your skin is simply trying to protect itself.

Used shoes often don’t fit perfectly because they’re shaped to someone else’s feet. This creates constant rubbing and squeezing. As the article explains, “Your toe rubs against the inside of the shoe,” and over time “your skin thickens to shield itself,” forming a hard, yellow bump made of dead skin. Hard corns may feel painful, “like walking on a pebble.”

Less common causes exist. Intense rubbing can lead to blisters, and poor hygiene in second-hand shoes may spread fungi. However, fungal infections usually cause “itching, redness, peeling, or yellowed nails,” not a single raised bump.

Used shoes are riskier than new ones because “they might already be shaped to someone else’s feet,” and they can carry “sweat, bacteria, or fungi,” which increases irritation or infection.

Most cases improve with simple care: stop wearing the shoes, soak your feet, gently file thick skin, and protect the area. Avoid cutting it yourself. If it becomes painful or infected, see a specialist. A yellow bump is your skin’s way of saying “enough friction!” Fixing the shoe fit usually solves the problem.