Gel permeation chromatography (GPC) is a technique used to separate molecules based on their size. It involves passing a liquid sample through a column filled with porous beads. Smaller molecules can enter the pores of the beads, while larger molecules cannot, allowing them to elute from the column faster. GPC is commonly used in the analysis of polymers and proteins. By measuring the time it takes for different molecules to pass through the column, scientists can determine their molecular weight and size distribution, which are important for understanding the properties of materials.