Stanislav Krykun, CEO of Polish packaging company DST-Pack, explains that when a user takes the first sip, liquid leaves the cup, causing internal pressure to drop. The small hole immediately allows air to flow in, filling the void and maintaining a continuous flow.

"Without this air vent, reduced internal pressure would create a suction force, holding the liquid back. When a user tries to tilt the cup, air would rush strongly through the drinking opening, causing a sudden pressure change that makes the liquid spill out," Krykun explained.

Beyond maintaining a steady flow, designers position this small hole opposite the drinking spout to prevent air and liquid streams from obstructing each other. The hole also allows steam to escape, helping reduce the temperature of the first sip of coffee to prevent burns.

Takeaway coffee cups are designed with two symmetrical holes, providing a safe and smooth drinking experience. *Photo: Reader's Digest.*

Previously, early 20th-century coffee cup lids were sealed and prone to leaks. By the 1970s, as takeaway drink culture became widespread, many customers would puncture lids themselves to drink while walking or driving. To address accidents from hot liquid burns, manufacturers improved cup lids. The introduction of the Solo Traveler lid series in the mid-1980s, featuring a raised spout and a small air vent, marked a significant milestone.

In addition to the air vent, modern cup lids incorporate a shallow recessed groove around the rim, acting as a mini drainage system. If coffee splashes, this groove guides the liquid back into the cup instead of spilling out.

Today, manufacturers continue to use this core design on plastic or biodegradable cup lids to ensure user safety.

By Bao Nhien (via Reader's Digest)