From looking outside to an input value

Take the first clue: what is happening outside.

The simplest version uses an on-or-off input:

no rain → 0

rain → 1

But rain is not always just absent or present. It can be light, steady, or heavy.

An input value can preserve those differences by using numbers between 0 and 1:

no rain → 0

light rain → 0.2

steady rain → 0.5

heavy rain → 1

A could provide the measurement.

"Look outside" names the source. The number that represents the measurement is the input value.