Simple inventions

Bert pointed to the Hippo Roller. A simple method for transporting water in low tech areas.

It is interesting how effective inventions are often really simple. Recently I bought a new shower, and it has little rubber spouts where the water rays come out, which means that any accumulation of lime scale can just be rubbed off. Genius.
Where I live now it's less important, but back in flat Denmark it would be a blessing. Here, I used an electric kettle for years, and there was barely any lime scale to speak of (a see-through layer). In Denmark, use one for a few months, and there's a pencil-thick layer! I've seen kettles where the bottom with the coil was practically one big block of lime scale. (Which is dumb, since obviously it insulates.)