Q "All cows eat grass," I was taught in music many years ago. Grass contains chlorophyll, which is based on the only mineral grass contains, magnesium. So why do people drink milk for the calcium it is said to contain? asks Tony Hunting

A Magnesium has many roles in a plant – including in chlorophyll molecules (the biological pigment needed for photosynthesis in green plants), where a magnesium ion sits in the central cavity of the large ring-shaped part of the structure. However, it is not the only "mineral nutrient" in plants.

Besides phosphorous, potassium, sulphur and other nutrients, plants also contain calcium that is used, among other things, in the cell walls of a plant.

Cows munch on grass and other plant matter and their digestive processes break it down and allow the cow to absorb a proportion of the nutrients within. Magnesium and calcium play important roles in the body, including in muscle contraction and skeletal development. After a cow has had a calf, it produces milk to feed its offspring and help it grow and develop. Nutrients are carried in the blood to the mammary glands where milk is produced. In fact, milk does not only contain calcium; apart from fats and proteins, it has a host of other components, including vitamin B12, potassium, zinc and magnesium. In modern society we have harnessed this milk production to produce copious amounts of the white stuff for our supermarket shelves.

Q I have been reading a book that suggested the possibility of the existence of parallel universes. Is this theory still considered to be too "far out" to be generally accepted? asks George Lange

A As Dr Daniel Mortlock of Imperial College London tells me: "Probably the strictest definition of parallel universes relates to the 'many worlds' interpretation of quantum mechanics, which imagines that all the possible results of every decision, measurement, etc are realised in one of an infinity of parallel universes." However, such a view is not championed by all.

"This many worlds interpretation of quantum mechanics is certainly not generally accepted by the world's physicists, albeit not for the usual scientific reason (ie, that it makes predictions that aren't supported by experiment), but for the more philosophical reason that its only predictions are those that were already made by quantum mechanics," Mortlock says.

"There is hence lots of debate about whether this is really a theory at all or just an interpretation of quantum mechanics."

There is, however, another meaning attached to the concept of parallel universes: the theory of multiple universes, or the multiverse. "This is much more like a standard physical theory: our universe is part of some much larger construction in which, for example, separate 'bubble' universe regions are formed, possibly with different physical constants, etc," says Mortlock.

"In some versions of these theories the expanding bubbles can collide. This theory is hence potentially testable – the detection of a 'bubble collision' signature would represent strong evidence for the multiverse."

While the multiverse theory has also not been generally accepted, it has not been completely ruled out. "The most compelling arguments in favour of the multiverse are theoretical: one is that, if the different universes have different physical constants, it's not such insanely good luck that our universe happens to be just right for life to exist.

"Another is that the theory of inflation (the best model we have for the first instants after the big bang) very naturally predicts that many bubble universes form," says Mortlock. "It is actively being worked on by many well-respected theoretical physicists and one of the main suspects for a fundamental description of reality."

Q Why do liquid molecules exert force in all directions (asked in relation to the buoyant force exerted by water in an upward direction)? asks Amitabh Saran

A First, it's worth noting that pressure is simply a measure of force per unit area. Now, let's imagine a beaker of water sitting at rest on a table top. At any point in the beaker, the pressure acting on that point is the same in all directions – this is because unlike a solid, the molecules in a liquid can move past each other. What's more, any point in the beaker, at the same depth, will experience the same pressure. However, the pressure increases with depth as a result of the increasing volume, and hence the increasing weight, of water bearing down.

Now let's put a cube of wood into our beaker of water. The bottom of the block is deeper in the beaker than the top and so is experiencing a greater pressure than the top of the block. But the pressure acting on the different sides of the block is the same at any given depth, so there is no net pressure to move the block left or right.

But where does buoyancy come in? Well, as the pressure acting on the bottom of the block is greater than that acting on the top, it follows that the force acting on the bottom of the block is greater than that acting on the top, so the net force is in the upwards direction. This net force is equal to the weight of water displaced by the block. And the volume of water displaced is equal to the volume of the block submerged in the water. It was this revelation that got Archimedes so excited he allegedly jumped out of his bathtub shouting "Eureka!"

If the weight of the block is greater than that of the water it displaced, the block will sink, whereas if it is lighter in weight, the block will bob up –like a floating plastic duck, left.

Q Has the evolution of humans slowed down or stopped? asks Anna Leoni

A As Dr Chris Tyler-Smith of the Wellcome Trust Sanger Institute says, humans still face selective pressures, though these have changed. "Particularly in the westernised world, in the developed world, infectious diseases with some exceptions, in general are less of a selective force," he says.

But this has not always been the case. "For most of our evolution, and unfortunately still in some parts of the world, they are one of the major selective forces."

But there are other factors that affect who passes on their genes. "I think things like mate choice and family-size choice will now be influencing our evolution still, even in developed countries.

"Something like choice of family size is something that has not really been much of an option for most of our evolutionary history. Mate choice in one form or another has probably been a major force throughout and continues to be."

However, measuring how quickly we are evolving is a far from straightforward.

"It is tempting to think that evolution is slowing down as some of these pressures are changing, but I don't know how we would really quantitate that unless we can wait for a million years to pass and then look back," he says.