Absorbency is how well a material soaks up liquid. A paper towel is very absorbent — it quickly soaks up spilled water. A plastic tray is not absorbent at all — water just sits on top. Materials with many tiny spaces, holes, or fibers tend to be more absorbent because the liquid gets pulled into those spaces. Absorbency is an important property for choosing materials — we use absorbent materials for towels and sponges, and non-absorbent materials for raincoats and food containers.
Drop the same amount of water onto different materials: a paper towel, aluminum foil, a cotton ball, a piece of plastic, a sponge, and a sheet of wax paper. Observe which ones soak up the water, which ones let it bead up on the surface, and which ones absorb fastest. Measure how much water different materials can hold using a simple drip test.
Spill some water on a table and you will probably reach for a paper towel. Press the paper towel onto the puddle and the water vanishes from the table — it got sucked up into the towel. This property is called absorbency, and it describes how well a material soaks up liquid.
Absorbent materials are full of tiny spaces that liquid can flow into and be held. Paper towels are made of thin fibers with gaps between them. When water touches those fibers, it gets pulled into the gaps and spreads through the material. Sponges work the same way — they are full of tiny holes and channels. Cotton fabric, wool, and newspaper are also absorbent. The more spaces a material has, the more liquid it can hold.
Non-absorbent materials do not have those tiny spaces, or their surface repels water. Plastic, glass, metal, and wax paper are non-absorbent. Drop water on a plastic cutting board and it just sits there in a puddle — the smooth, solid surface gives the water nowhere to go. Wax paper actually pushes water away; the waxy coating makes the surface water-resistant. That is why wax paper is used to wrap sandwiches — it keeps moisture in without soaking it up.
An important thing to understand is that absorbent materials do not destroy the water. The water is still liquid water, just held inside the material. Squeeze a wet sponge and the water comes flowing out. Hang a wet towel on a rack and the water slowly evaporates out. The absorption is temporary storage, not destruction.
People choose materials based on absorbency all the time. Towels and mops are absorbent because their job is to soak up water. Raincoats and umbrellas are non-absorbent because their job is to keep water away from you. Diapers are designed to be extremely absorbent, using special materials that can hold many times their weight in liquid. Food containers are non-absorbent so the food's moisture stays inside and outside moisture stays out. The next time you pick up an everyday object, ask yourself: does this need to absorb liquid, or does it need to repel it? The answer explains why it is made of the material it is.