When you suck, the pressure inside the straw will be less than the outside atmospheric pressure, and water will be put into your mouth under the action of the outside atmospheric pressure, or think about it from another angle. If you take a drink, you will feel that your mouth will be sucked in. If you make a hole behind the cup now, air will be sucked in. Imagine a cup with holes as a straw, because there are holes on the same side, and then imagine the air as a drink, which is much easier to understand.

The material of straws is mainly polypropylene, and straws produced by regular manufacturers need many processes. The straw made of qualified polypropylene can withstand the high temperature of about 80℃ and will not emit plastic smell when heated. There are also illegal vendors who use recycled plastic products to make mixed plastics, and the quality of the products is difficult to guarantee.

Extended data:

After the plastic melts, it passes through the extrusion die, which is called the die, and then the machine extrudes the plastic like a syringe (this process is continuous, and there is a screw on the machine, which will be extruded at a certain speed). When the plastic is still softening, it is sent to the machine, directly molded, and then cut by the machine, which becomes the straw we see.

"Three noes" straws can be processed from recycled waste plastics. Once exposed to high temperature, they will produce toxic substances and endanger health. There must be something wrong with the quality of the "three noes" plastic straws. Because they are made of inferior or recycled plastics, pigments may be used to cover up impurities. Once you use it to drink fruit juice or hot drinks, harmful substances will dissolve with it, thus causing harm to the human body.