Neutrinos are probably the strangest particles in the universe. They are commonplace in nuclear reactions, especially at the sun's core.

Billions are harmlessly passing through you as you read this. They hardly interact with matter at all. A neutrino could pass through many million million miles of lead before it would be stopped.

The neutrino was predicted long before it was discovered. It was found to come in three types.

The ghostly neutrino may be one of the most important parts of the cosmos. For years scientists believed that neutrinos had no mass. But some experiments have suggested that they may have a tiny mass.

Even a tiny mass would be important because there are so many neutrinos in the universe. They could make up the majority of the universe's mass. In a sense the universe would consist of neutrinos and a smaller number of other particles.

Announcing their discovery at a conference in Japan, an international team of scientists has produced the best measurement of the tiny mass of the neutrino. It also has evidence that neutrinos change between the three types as they travel in space.

The results come from two years of observations made with the Japanese Super-Kamiokande neutrino detector.

This is a huge tank holding 50,000 tonnes of purified water situated underground in an old zinc mine.

As billions and billions of neutrinos stream through the water, only about five each day interact with the water producing a flash that can be detected by 11,000 sensors on the inside of the water tank.