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The Independent Online
Q) Why is yawning "infectious"?

A) There are three theories to this common yet complex phenomenon. The physiological theory suggests we yawn to get more oxygen into our systems. Yawning is thus "infectious" because everyone in any given room is likely to be short of fresh air at the same time. The evolutionary theory says we yawn to display our teeth. Yawning acts as a warning to others but has lost its aggressive meaning. The boredom theory says that if everyone finds something boring they will yawn.

Experiments suggest the physiological theory is incorrect. The evolutionary theory is difficult to test. The boredom theory holds some substance but creates more questions. Subjects yawned more when they watched a still pattern for 30 minutes instead of a rock video. But did they yawn for psychological reasons (boredom) or physiological reasons (boredom made them sleepy)?

No one is really sure. During the day, most yawns occur about an hour before sleep and an hour after waking. There is an unmistakable link between stretching and yawning. Perhaps yawning and stretching were once part of the same reflex, yawning being a stretch of the face. We are sensitive to any cue that suggests yawning. Even reading this answer may prompt you to yawn.

Q) When you photograph some flowers, why do they appear different colours in the final prints?

A) Some plants (bluebells, for instance) have very high infra-red reflectance, which we don't see because our eyes do not detect these wavelengths. The red-sensitive dye in some photographic film, however, does detect the infra-red to some extent. So when the film is developed there can be a red or pinkish tinge to your bluebells.

Questions and answers are provided by Science Line, whose dial-a-scientist service is on 0345 600 444.