He is doing special exercises to strengthen the interosseous muscles of his hand.

She was a secret kunoichi (female ninja).

He threw a handful of sand into the sea.

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

He missed a once-in-a-lifetime opportunity.

He was focused on his work all day long.

One day, he suddenly decided to go on a journey.

No one knows what he did to spend one day.

This day was a very rare day for me.

I couldn't get in touch with her for a week.