The Case of the Leaky Travel Mug, Part 2

I wrote previously about a travel mug I have that suddenly had a mysterious leak. Well, I am here to say that the mystery has been solved. And you know what, it wasn't terrorists after all. Or robot ninjas. Or terrorist robots. Someone else had the same experience and shared his findings with me, and I decided to test them out myself.

As you can see in this wonderful illustration (aren't I an amazing artist?), while most of the mug is one solid piece of aluminum (well actually I think it's two, but they're very well connected and there's no way that seam was the source of the leak) the bottom is a separate piece. It's just a piece of round plastic with a foam piece stuck on it, that fits nicely in the bottom.

Most of the time, that is. We all learned way back in school that when metal gets hot, it expands.  So when I hand-washed it in the sink with hot water, the metal expanded and the bottom piece no longer had a tight seal. And while it was sitting there in the sink water was able to get inside. When I washed mine last night I could even see air bubbles coming out of the bottom from the water entering as I put it in the rinse sink.

Then when it was sitting in the dish rack waiting to be dried it was sitting upside down, and the water that got inside was unable to drip back out. So when the metal cooled and shrunk back to normal the bottom piece once again had a snug fit and the water was trapped inside. This time I could even hear water sloshing around inside when I shook it.

This evening, I set it out on the counter and got some water boiling. When I poured the boiled water into the mug, my counter was instantly covered in a puddle of water. The boiling water caused it to expand so fast the trapped water was able to escape immediately.

But on the morning of the initial experience, I had coffee that wasn't near boiling temperature, but still hot enough to expand it enough to let it leak slowly. And as I finished that mug of coffee the bottom cooled and closed up and the foam on the bottom had dried by the time I went for a second mug of coffee. The second time, it once again expanded and let the remainder of the trapped water out. But when I did my tests later on in the day, they didn't do anything as there was no water left to leak out (plus my first test with cold water wouldn't have caused it to expand anyway).

So it turns out there was a simple scientific explanation after all. I'm so glad the world is not actually in danger from terrorists out to ruin my coffee drinking experience. Although I still blame them for the peach I ate today being a little riper than I had hoped.