Why Evaporated Milk Won’t Pour

As the girls spend time with their grandparents, Pearl stumbled on a curious kitchen mystery. She noticed someone had punched a hole into a tin of evaporated milk, but the milk was barely coming out. So, seeing that the milk was not flowing as it should, she tilted her head, confused.

“Why isn’t it pouring properly?” she asked.

I told her, “You need to punch another hole on the opposite side.”

Still puzzled, she asked why. I gave her a quick answer, but I could see that her 7-year-old mind didn’t fully understand. Maybe I didn’t understand the phenomenon as I should, validating Albert Einstein’s claim. To test that claim, I have decided to share the longer version here. You be the judge of my understanding of this principle of physics.

So, the reason a second hole is needed boils down to air pressure. Specifically, how fluids behave in enclosed spaces.

When you punch just one hole, the milk tries to flow out. But there’s a problem: as the milk leaves, nothing is replacing the empty space inside the can. There is no vacuum in nature. In nature, when something leaves, something must replace it. That inability of milk to be replaced by anything in the tin, this time by air, creates lower pressure inside, while the pressure outside the can (atmospheric pressure) remains high. This imbalance resists the flow of the liquid. Milk just refuses to leave when there is nothing to replace it.

The result? The milk trickles out slowly, or “glugs” and sputters as air bubbles struggle to sneak in through the same hole. Every drop of milk that falls down must have been replaced by air.

But if you punch a second hole, air can now enter freely through one side while milk flows out the other. This keeps the pressure inside equal to the pressure outside. And the milk flows smoothly.

This everyday kitchen moment is actually a textbook example of a principle called fluid dynamics which every secondary school physics student should know, at least in theory. It’s so important that it shapes some of the most essential systems in our lives.

This principle is actually something you can use to neutralize those who say ‘wetin school give us’. This is because many everyday systems that we take for granted depend on pressure flow and just like in the milk can, there are consequences when that balance is disrupted.

Let’s start with breathing. When we breathe in, our diaphragm contracts and expands our lungs. This increases the space in our chest cavity, lowering the pressure inside. Because outside air is at a higher pressure, it rushes in to fill our lungs.

But what if it didn’t work? What if your lungs can’t create that pressure difference, for instance, due to a collapsed lung (a condition called pneumothorax)? Air doesn’t flow in. Breathing becomes labored and you will die if there is no medical intervention which applies that principle. Just like the milk stuck in the tin, air won’t come in unless there’s space and pressure to allow it.

It’s the same with airplanes in flight. On a recent flight, I sat by the window side and observed the shape of a plane’s wing. The wing is designed in a way to cause air to move faster over the top than underneath. This creates lower pressure on top and higher pressure below, making it generate lift that allows the plane to fly.

But what if pressure is disrupted? If the airflow is blocked or turbulent, lift is lost. Planes can stall, dip, or even crash if the pressure difference is not properly maintained. Like milk that can’t pour because of poor airflow, planes need that consistent pressure differential to stay airborne.

My last example is of Volcanic Eruptions. What you see on television is nature’s way of balancing itself. Deep underground, magma contains trapped gases. As magma rises, pressure builds. If that pressure finds a safe outlet (like small fissures or vents), it’s released gradually.

But if it does not and there is no release, then disaster strikes. Pressure keeps building until it violently forces its way out, resulting in a volcanic eruption. The eruption is nature’s version of punching a second hole — but too late and too forcefully. That’s what happens when trapped pressure has no way out.

As these three examples show, this principle governs the world around us:

For anything to flow, whether it is air, liquid, or even lava, there must be a path in and a path out.

So, the next time you open a can and make that second hole, you’re not just helping the milk flow. You’re aligning with the laws of nature, physics, and engineering. And now you understand exactly why you are doing what you are doing.