Here is the control panel of a counter-top cold/hot Tami 4 water bar. Push a button – you get hot water. Push another – here’s a refreshing cup of cold water.
Of course, that’s why the buttons – all seven of them, for there’s a lot more you can make this glorified water faucet do – are labeled! The problem is, only one – wisely perhaps, the one labeled “Extra hot” – can be easily read. The others have the label printed on mirror-like shiny surfaces in light gray, which means you need just the right angle of lighting and viewing to make the text out at all. See the photo – the legends on the right are visible (though at very low contrast) while of the two on the left the top one is just barely readable and the bottom one is totally invisible.
They say writing was invented in the fertile crescent in the 4th millennium BC… but we had to wait till our day and age to invent invisible writing.