Preface–

Eyes are something very unreliable. They show us the world around us but they can't let us see everything.

There are so many invisible things on this earth.

Music – Invisible, but you can hear it.

A flower's aroma – Invisible, but you can smell it.

Warmth – Invisible, but you can feel it.

See, there are so many things, which our eyes can't show us. Good to have other senses.

But is there perhaps something which neither can be seen, nor heard, smelled or even not felt? Something completely invisible?

Turn off the lights. – Can you see anything?

Listen closely. – Can you hear anything?

Inhale deeply. – Can you smell anything?

Comb with your hand through the air. – Can you feel anything?

No?

But although your senses can't tell you what's going on around you, you know what this means.

Night is perfectly invisible, but still, we know that it is present.