Porous means full of tiny holes or spaces that let liquids or gases pass through. A sponge is porous: when you dip it in water, the liquid flows into all those little holes and spaces throughout the sponge. Paper towels are porous too, which is why they can soak up spills instead of just pushing the liquid around.

Think about the difference between a porous material and a non-porous one. A brick is porous: if you leave it in the rain, water seeps into it. A piece of glass is non-porous: water just runs off the surface. Your skin is actually porous, which is how you sweat: tiny drops of moisture can pass through microscopic openings called pores.

Scientists use the word porosity to describe how porous something is. Sandstone has high porosity because it has lots of space between its grains, which is why water and oil can flow through underground rock formations. Engineers designing water filters rely on porous materials that let clean water through while trapping dirt and other impurities.

The word can also describe something with weaknesses or gaps, like a porous defense in basketball that lets opponents slip through easily.