Was planning on a road bike ride yesterday morning. Got to the bike and the tire is flat. I went to the gym instead. When I got back I pumped up the tire and figured I'd fix the flat at night.

I go to the bike 24 hours later and the tire is fully inflated.

How does that happen? Over my 20 years of mountain biking / road biking I've seen this a handful of times. But never really understood why.

I'm guessing it has something to do with the presta valve.