I recently had this in regards to roll-on deodorant and ballpoint pens. If there's ink/deodorant available, rubbing it on paper/skin will deposit it. If there

I recently had this in regards to roll-on deodorant and ballpoint pens. If there's ink/deodorant available, rubbing it on paper/skin will deposit it. If there's no ink/deodorant available, this causes friction on the rollerball causing it to rotate and reload the liquid. Such a simple concept but hard to get right (at least initially) - Ball too big, concept doesn't work. Ball too small, it'll leak everywhere. Someone came up with the idea in 1888, but it didn't really work out until 1943. Neat. https://en.wikipedia.org/wiki/Ballpoint_pen https://en.wikipedia.org/wiki/Ballpoint_pen And of course there's a whole other can of worms in what it took for me to be able to express this wonder to people all over the world via this comment