A soda straw or simply straw is a long, hollow, thin-walled tubular stalactite less than about 1 cm in diameter. It grows from the ceiling straight downward.
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. And they are formed by dripping water.
Before reaching the cave, the water went through limestone rocks and had time to dissolve small amounts of this limestone. But the ability to dissolve limestone depends on the CO2 in the water, which forms carbonic acid with water. The weak acid, think about carbonated water, is able to dissolve a small amount of limestone. When reaching the cave the whole process goes the other way round, the CO2 leaves the water and vanishes into the cavern air and so the water looses the ability to keep the limestone dissolved. In chemistry this is called a supersaturated solution. The limestone precipitates, 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 the weight of the water gets too large for the surface tension and the drop falls down. But in the meantime the water looses carbon dioxide (CO2) to the air 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 that's 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, forming a drop at the end of the tube, depositing another small amount of calcite, and falling down. This process continues until the water flow from the crack ends or the tube gets blocked by calcite crystals or some dirt.
Sometimes the straws become long enough to break because of their own weight. Or they are broken by an air stream called cave wind. But both is really rare, and they can become several meters long. In most caves the straws reach the bottom first.
Straws are normally dry outside, because the water runs inside. If there were water outside they would not stay thin, 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 tube and seal it. And sometimes the water just changes its path. The feeding crack is not a single spot, but it's 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.