Kou Sennen is my best friend.

Photochemistry is a branch of science that studies how chemical reactions proceed under the influence of light.

The photoelectric effect is a phenomenon where a material emits electrons after absorbing light.

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.