Just before eating my breakfast at a local restaurant, I fancied testing some scientific principles related to pressure and temperature differentials and buoyancy. First, I inserted my straw into the hot coffee and blocked the top of the straw with my finger. In this way I was able to remove the straw with a column of hot coffee inside it leaving the bottom open. Some people claim that this occurs because the gravity pulling down on the column of coffee produces a vacuum inside the straw, and the vacuum sucks the coffee into the straw preventing it from flowing out. This is not true. Like I have explained before, vacuums don’t suck. The pull of gravity tends to create a low pressure area inside the straw, and the push of the atmospheric pressure against the bottom of the column of coffee is enough to counter gravity and keep it inside the straw.
I then proceeded to place the tip of the straw with the coffee inside the cold milk. The hot coffee is less dense than the cold milk. Under the influence of a gravitational field, liquids that are less dense will float on top of liquids that are denser. You can see in the video that, even though the coffee is in direct contact with the milk at the bottom of the straw, the column of coffee inside the straw remains by and large unperturbed (with the exception of some coffee at the coffee-milk interface mixing with the milk due to equilibration of the temperatures of the liquids). Then I did the opposite. I placed a column of milk inside the straw in contact with the coffee. Because the hot coffee is less dense than the milk it starts flowing up the straw almost immediately, while the cold milk that is denser flows in the opposite direction.
After this I ate my breakfast: scrambled eggs with bacon and home fries and blueberry toast (with no butter).