"Once in a lifetime" means to cherish the encounter that happens only once in one's lifetime.

During the dry season, it is important to conserve water.

What is the half-life of this substance?

His dog is currently in its sexually active period.

Mothers in the lactation period should strive for a well-balanced diet.

Apologizing at this point is too late.

This is the dawn of a new science.

The story of the time of creation is mysterious and interesting.

A new school term is starting.

He experienced many difficulties during his adolescent.