Because that is how it cleans. The foam is an extremely fine abrasive, so every wipe takes away a little of the block along with the mark. A block that came back unchanged would not have cleaned anything.

Heavier marks and rougher surfaces use it up faster. Treat it as something that is consumed, like a pencil eraser, rather than as a sponge you keep.

One thing does tear it before its time: wringing it out. Melamine foam has almost no strength against a twist. Press it flat between your palms instead.

Ballpoint pen, pencil and crayon on matt emulsion walls. Scuffs on shoe midsoles. Grease, tea, coffee and soap scum on tile. Marks on switchboards, kitchen surfaces and bathroom fittings.

The pattern is that it lifts marks sitting on top of a surface.

Permanent marker. And anything that has soaked into a material rather than sitting on it, such as dye in unsealed grout, oil in raw wood, or old stains in bare plaster.

Micro-abrasion only reaches the surface. If the mark is in the material rather than on it, rubbing harder will not lift it. It will only use the eraser up faster.

Yes. The foam cleans by micro-abrasion rather than by detergent, so wetting it is all it needs.

How wet matters more than people expect. Melamine foam is open-cell and takes up a lot of water for something that weighs two grams, and a dripping block slides over a mark and pushes dirt around instead of lifting it. Press nearly all of it out before you start. Damp, not wet.

The full list is further up this page, under where to use. It is worth reading rather than skimming, because it has three parts and the middle one is the honest answer that most listings leave out: surfaces nobody has tested yet.

The short version is that abrasion dulls a finish. Test a hidden area first, every time, on any surface you care about. Not for use on skin.

Keep it out of reach and do not use it on skin. It is a cleaning tool rather than a toy, and the foam breaks into small pieces as it wears, so keep those out of reach too.