A full device stops taking updates, which quietly makes it less safe as well as slower. Photos and video are nearly always what filled it — look there first.
The drawing that proves the sentence above. It is the same card that is in the book.
Think of it as
a closet so packed the door won't shut.
For example
The phone refuses to take another photo. You find one vacation video from last year and delete it, and there is room for hundreds more photos.
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.