Research which measured brain patterns using electroencephalogram readings showed that bottlenose dolphins slept by alternately shutting off each side of the brain.

Studies showed that they closed down one half of the brain together with the opposite eye when sleeping.

The active side of the brain stays on the lookout for predators and obstacles.

It also monitors the dolphin's physical conditions and tells the animal when to go to the surface for air.

After about two hours, the active side of the brain shuts down and the sleeping side takes over and keeps watch.

Dolphins sleep in this fashion for about a third of their day.

Several types of dolphin sleeping behaviour has been observed. They either swim slowly and surface occasionally for a breath; rest at the surface with their blowhole exposed; or rest on the bottom of shallow water and rise to the surface every now and then for air.

Scientists think that behaviour while sleeping depends on circumstances and individual preferences.

Does making cracking sounds by pressing your knuckles cause any harm?

Some people, especially schoolboys who enjoy making loud noises with various parts of their bodies, like to make cracking sounds by sharply pressing on their fingers and knuckles.

The pressure inside a joint is reduced when you bend your finger.

With a sudden loss in pressure, dissolved gas in the fluid in the joint comes out of solution and into the joint cavity.

This makes a cracking sound.

Habitual joint cracking is not thought to be related to arthritis, as is commonly thought.

But repeated cracking has been associated with loss of grip strength, especially with age, and soft-tissue swelling.

What happens when lightning strikes water? Do all the fish die?

When a bolt of electricity, such as a lightning bolt, hits a watery surface, the electricity can run to earth in a myriad of directions.

Due to this reason, electricity is conducted away over a hemi-spheroid shape which rapidly diffuses any frying power possessed by the original bolt.

If a fish is directly hit by lightning, or close to the impact spot, it could be killed or injured.

However, a bolt has a temperature of several thousand degrees.

It could easily vaporise the water surrounding the impact point.

This would create a sub-surface shock wave.

The shock wave could kill a fish or deafen human divers over a far wider range - tens of metres.

Graphic: SPBRAIGLO