Third tube puncture in 3 weeks I have the 3-rd puncture of the rear wheel tube during 3 weeks. All of them have the same pattern: 1 small breach about 0.5mm long. I added the photographs of the last one.

Bubble inside the water coming out of the tube through the puncture.


The little puncture.


Previous patched puncture


After the second puncture I thought, that it was caused by a grain of sand that got between the tube and the tire. So when I assembled the wheel I carefully cleaned the space between the tube and the tire with open hand.

Now I have no idea what can cause these punctures and what should I do to avoid them?
I found it! After careful inspection of the tire I found a small piece of glass stuck into it:



It's sharp side was facing the tube.

Here is the place where it was stuck:



On the internal side of the tire you can see a little hole:



It was hard to feel this little thing by hand. But when I pumped the tire and rode, creating high pressure, this sharp edge has eventually punctured the tube.