Why does rain soak cardboard but roll off plastic?

Picture a rainy afternoon. A cardboard box sits by the door, and a plastic bucket sits right beside it. Raindrops tap on both of them, pat, pat, pat. If you touched them later, the box would feel soft and soggy, while the bucket would feel smooth, with drops sliding down its sides.

Both of them got the very same rain. So why does rain soak into cardboard but roll right off plastic? Let's look up close and find out.

Cardboard is made of paper. Look at paper up close, and you would find fibers, which means thin strands of wood as fine as hairs. Thousands of these fibers lie crisscrossed in a tangle, like a messy pile of string.

Fibers never fit together perfectly, so small gaps sit between them everywhere. Those gaps are too small for your eyes to see, yet they make cardboard a bit like a sponge. They leave room for water to get inside.

Water clings to fibers, the way a wet sock clings to your foot. So each raindrop creeps along the fibers and pulls itself into the gaps. The cardboard starts to absorb the rain, which means soaking it up inside.

Once cardboard absorbs water, its wet fibers swell up and let go of each other. That is why a rainy box turns soft and floppy. Now think about the plastic bucket. What do you guess plastic is missing, so the rain cannot get in?

Plastic has no gaps at all. Its smallest parts pack tightly together into one smooth, solid sheet, with no fibers and no spaces in between. With nowhere to creep in, the water stays outside, so plastic cannot absorb the rain.

Water does not cling to plastic either. Instead, each raindrop holds onto itself and pulls into a round little bead. That is why a plastic raincoat looks covered in shiny beads during a shower.

A round bead has almost nothing holding it to the plastic, so it slides and rolls downhill. Drop after drop rolls away, and the plastic underneath stays dry.

Two words to remember: fibers and absorb. Rain soaks cardboard because water clings to its fibers and creeps into the gaps between them, but plastic has no gaps and nothing for water to cling to, so the rain rolls off.

Next time rain taps on the window, you will know just where every drop is going, so snuggle in, warm and dry.