Storing food is common in members of the crow family. A new study tested the birds outside for this naturally occurring behavior, which may have evolved（进化）specifically because it gives crows a survival advantage. Some crow species are known to naturally use tools to recover food. So the researchers tested whether the birds could store and recover a tool so they could use it to find their food after a gap of 17 hours — something we wouldn’t expect them to do naturally. But they were able to instantly select the tool out of a number of unnecessary items.

In another experiment, the researchers taught crows to select a token（礼品券）from a number of items so that they could then exchange it for food. Again, the birds then showed that they could plan for the future using this new behaviour. This is different from all of the previous studies in future planning, which have focused on naturally occurring behaviour. For example, we know that chimpanzees select, transport and save appropriate tools for future needs.

These studies have shown that animals can plan for the future — but they leave an important question open for debate. Are animals only able to plan to use abilities that have evolved to give them a specific advantage, or can they flexibly and intelligently apply planning behaviour across various actions? Most critics would say the former, as the animals were tested in naturally occurring behaviour.

But the new research provides the first evidence that animal species can plan for the future using behaviour that doesn’t typically occur in nature. This supports the view that at least some recognitive（认知的）abilities in animals don’t evolve just in response to specific problems. Instead, it suggests that animals can apply these behaviour flexibly across problems in a similar way to humans. We need to study how flexible behaviour evolved. Then we might be able to see how crows’ ability to plan for the future fits in with their broader cognitive powers.

   As the pace of life continues to increase, we are fast losing the art of relaxation. _______ you are in the habit of rushing through life, being on the go（忙碌）from morning till night, it is hard to _______. But relaxation is _______ for a healthy mind and body.

Stress is a（n）_______ part of everyday life and there is no way to _______ it. In fact, it is not the bad thing that it is often supposed to be. A _______ amount of stress is important to provide motivation and give _______ to life. It is only when stress gets out of control that it can lead to poor performance and _______ health.

The amount of stress a person can bear _______ very much on the individual. Some people are not afraid of stress, and such a ________ is obviously chief material for managerial responsibilities. Others lose heart at the ________ of unusual difficulties. When exposed to stress, in whatever form, we ________ both chemically and physically. In fact we make a choice between ________ and fight. And in more ancient days the choices made the ________ between life and death. The crises we meet today are ________ to be so extreme, but however little the stress, it requires the same response. It is when such a reaction ________ long, through continued exposure to stress, that health becomes ________. Such serious conditions as high blood pressure and heart disease have ________ links with stress. Since we cannot ________ stress from our lives we need to find ways to ________ it.

So what do you think of stress? What is your way to deal with it?