Which waterproof, luminous marker should I use to name and label dark flip-flops? Should the marker cover the dark background? Sauna?

It's about visits to the sauna. Which pen is best for marking dark flip-flops with a name? The pen should be waterproof and luminous or cover the dark background with a light color. Preferably, the name or marking should be on the inside of the flap.

How to make air-drying modeling clay waterproof?

Help! I made a teacup out of TEDI air-drying modeling clay. Now I've discovered that it's not waterproof! Is there any way I can save the cup by making it waterproof? Does anyone have any last-minute ideas? It's supposed to be a Christmas present for my dad, so I'd appreciate some quick answers. Thanks!

iPhone waterproof?

Hello! I own an iPhone XR. However, I sold it used, thinking it was waterproof. I've been walking around with the iPhone in heavy rain every day for months now, and nothing has happened so far. Do you think it was just luck that the rain never got inside, or is it waterproof despite being…