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Cobras are always very alert.

alert being quick to notice things going on around you

The alert guard perceived a dim shape in the distance.

Such an alert man as he couldn't have failed to adapt himself to the new situation.

They took turns at staying awake and being alert to keep watch while the others slept.

So you mean they work harder and are more creative because a power-nap – a quick sleep – makes workers feel refreshed and more alert.

When you’re driving, you must stay alert.

a bomb alert The army was put on (full) alert as the peace talks began to fail.

Our competition is alert.

to alert somebody to something

I'm not ​feeling very alert today - not enough ​sleep last ​night!

You have to be very alert if you are driving at night in heavy rain / Someone had better alert the police; we've been robbed.

Suddenly he found himself awake and fully alert. Two alert scientists spotted the mistake.

Remember to stay alert and don’t let anyone in without their ID cards.