Many examples of a break in sustained focus can be seen in a ballet class for three year old children.
One need only to watch a ballet class for 3-year old children to see wonderful normal examples of breaks in sustained focus.
The kids at that age can follow instructions for short periods of time (minutes) and then they wander off into their own little worlds. They vacillate in and out repeatedly throughout the lesson.
It is easy to recognize the kids who have temporarily lost focus.
They will be crawling along the floor or climbing up the wall or wandering around when everyone else is lined up "following" instructions.
Here are some examples of activities on an older level that involve sustained focus :
1. Ask a person to count back from 100 by 3's;
2. Tell me the day of the week that is two days before the day I say.