Internet billed by how much you use, not how long. Video drains it fastest, and apps keep spending it in the background unless you go in and tell them not to.
The drawing that proves the sentence above. It is the same card that is in the book.
Think of it as
buying water by the bucket instead of the tap.
For example
You watch three movies on the train and get a message saying you have used your allowance. At home the movies cost nothing extra. On the train, each one was bought.
This is one card out of 275.
The hundred words you meet most are free — the whole set, animated, in one file that works with no internet.
Volume 1 has all 275 in fifteen chapters, with Staying Safe the largest chapter on purpose.