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Anyone who has carried coffee through a busy café, knows how easy it is to spill the drink over the sides of the cup. 

But carrying beer - even if you have had one too many - is much easier.

Now scientists think they know why, and it's all to do with the beer's foamy head.

Researchers studied waves created by people carrying different drinks to demonstrate that foam reduces sloshing, and dampens the waves themselves.

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Scientists have solved why carrying beer is easier than carrying a coffee, which spills more easily, using a moving platform (pictured). They found that beer with a foamy head (Guinness is pictured left and Heineken - with less foam - is pictured centre) sloshes around less in a glass than coffee (pictured right)

This knowledge could one day be used to work out better ways of transporting huge quantities of liquid more safely and efficiently.

Emilie Dressaire, an assistant professor of mechanical and aerospace engineering at Princeton University, set out to prove the common observation that coffee is tricky to carry, whereas beer is much easier.

She used a high-speed camera, a moving stage, and fluid with, and without, a head of foam.

Together with her team from Ecole Normale Superieure de Cachan, France and New York Univesrity (NYU), she studied the waves created upon a sudden movement. 

The team of researchers studied the waves created upon a sudden movement. Once they realised that the foam head on beer dampens the waves, they discovered that the thicker the foam (pictured right) the more damped the waves were (shown on the bottom row for different foam thicknesses)

Why does coffee spill more than beer? NYU explains

The researchers' measurements showed that foam, on Guinness and Heineken, reduces the sloshing and is very efficient at damping waves. 

In fact, just a few layers of bubbles are needed for the effect to work. 

They believe the foam is efficient because of viscous dissipation - where the 'thickness' or viscosity of a fluid takes energy from its motion and transforms it into heat energy.

So, as the waves form and travel, the foam on the beer rubs against the walls of the glasses and this rubbing costs a lot of energy, which results in damping.

The effect is not seen in cups of coffee, which do not have any foam on top of them.

The scientists presented their findings at the 67th Annual Meeting of the American Physical Society's Division of Fluid Dynamics, held in San Francisco this month.

While it might seem frivolous for scientists to study beer, Professor Dressaire said: 'There are different ways to approach a scientific or technological question and it's good to keep an open mind, because interesting ideas can come from the most familiar observations.

'Students are great at connecting their everyday life to some of the most difficult problems.

'To this point, their work has serious and promising applications in numerous industrial applications, such as the transport of liquid as cargo.'

Measurements showed that foam reduces sloshing, and is very efficient at damping waves. This image shows the average amount the different liquids (from left to right: Guinness, Heineken and coffee) 'sloshed'. In fact, just a few layers of bubbles were needed for the effect to work