Baking soda and vinegar followed by hot water.

This method utilizes a chemical reaction between baking soda (a base) and vinegar (an acid) to create a fizzing action. This fizz can help to dislodge and break down organic matter like grease and hair that commonly cause drain clogs. Following up with hot water helps to flush away the loosened debris. It's a popular first-attempt DIY method due to its simplicity and use of readily available, environmentally friendly ingredients.