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How to shrink a cotton t shirt
Prepare a large, boiling pot of water.
Clothing companies often stretch out their fabric so that less cloth is needed to manufacture a shirt. Washing the cotton shirt, or submerging it, in warm water will help the cloth in the shirt shrink back to its natural state.
If you want to avoid the hassle of buying a shirt that's too big and shrinking it yourself, look for pre-shrunk garments when you shop. Pre-shrunk shirts are increasingly common.