A soda straw or straw is a long, hollow, thin-walled tubular stalactite less than about 1cm in diameter.
Straws are dripstones pointing vertically downwards from the cave ceiling. They are very straight, thin and fragile. They are always hollow, the water is coming from inside, the outside is normally dry. They are formed by driping water.
Before reaching the cave, the dripping water went through limestone rocks and had time to solute small amounts of this limestone. But the ability to solute limestone depends of the CO2 in the water. Now the whole process goes the other way round, the CO2 leaves the water into the cavern air and so the water looses the ability to keep the limestone in solutiuon. The limestone gets solid again, forming small growing calcite crystals in the water.
A drop at the ceiling is formed by water coming down a crack. When the water drop grows, there is a point where weight of the water gets too large for the surface tension and the drop falls down. But in the meantime the water looses carbondioxide (CO2) to the atmosphere of the cave. A small amount of calcite crystallizes in the water.
The crystals are formed, where the CO2 gets lost: at the surface of the water. And they get deposited on every surface they get contact with. For a small drop on the ceiling, the surface gets contact in a circle around the drop, and thats exactly where the calcite is deposited. It forms a small circular rim, then a wall and later a tube.
The crack, where the water enters the cave, is inside the straw. The water runs through the tube it built itself, forming a drop at the top of the tube, depositing another small amout of calcite, and falling down. This process continues until the water dries out or the path of the water gets blocked.
Sometimes the straws become long enough to break because of their own weight. But thats really rare, as they can become severeal meters long. In most caves the straws reach the bottom first.
Straws are dry outside, because the water runs inside. And it is also necessary for them to stay thin. As soon as water runs outside, this water would deposit calcite on the outside, the tube gets conic, the straw becomes a stalactite.
This may happen because of various reasons, but the most common is the blocking of the tube. Sometimes a little dirt or sand is transported by the water and gets into the tube, where it deposits and blocks the path. Sometimes calcite crystals grow inside ttube and seal it.
And sometimes the water just changes its path. The feeding crack is not a single spot, but its a long crack along the ceiling. So if the water leaves the crack just a few millimeters beneath the old place, it will run down the straw on the outside.
For more info about stalactites see here.