Mental exercise is particularly important for young children.

No living things could live without oxygen.

Living organisms had existed on earth, without ever knowing why, for over three thousand million years before the truth finally dawned on one of them.

The output is way below last year's level.

Living is dear, accordingly we have to economize.

I have to tighten my belt.

Lifelong education means perpetual retraining.

For the first time in my life, I felt a pang of conscience, but there was no other way out.

He is by nature quite an easy-going man.

The instinct for survival is innate.