【答案解析】This may sound far-fetched, but tests have now been carried out which reveal that it is nevertheless the true explanation. Groups of new-born babies in a hospital nursery were exposed for a considerable time to the recorded sound of a heart-beat at a standard rate of 72 beats per minute. There were nine babies in each group and it was found that one or more of them was crying for 60 per cent of the time when the sound was not switched on, but that this figure fell to only 38 per cent when the heart-beat recording was thumping away. The heart-beat groups also showed a greater weight-gain than the others, although the amount of food taken was the same in both cases. Clearly the beatless groups were burning up a lot more energy as a result of the vigorous actions of their crying.