I put a glass bottle in a big pot, bottleneck up, I fill both pot and bottle with very hot water. The bottle is submerged except a few cm of the bottleneck and filled to the brim so some water inside the bottle is above the pot water.
Then I light the fire. After a while the water in the pot boils but even if I keep it boiling for 30 minutes the water inside the bottle does not boil. All I see in the water inside the bottle are very tiny bubbles.
The same happens if I put the bottle upside down (with some air at the top) What is the reason? Maybe the glass acts as insulator and water inside the bottle cannot reach 100 degrees? Or having a few cm of bottle (and water) above the pot surface cools the water inside?