A single path runs through the grid. It starts at the 1 and ends at the highest number, and it uses every field exactly once.
Numbers that follow each other always sit in fields that share a side - so left, right, above or below. Corner to corner does not count.
The 1 and the highest number are always printed already. That tells you where the path begins and where it ends.
Pick two printed numbers with a gap between them and work toward each other from both sides. Between 7 and 11 three numbers are missing, so you need exactly three fields.
Count first whether the way is long enough at all. If two numbers are four steps apart but their fields are six steps apart, the path cannot run there.
Example
For parents
What does this exercise practice?
The number path is the most accessible logic type in the book - the rule is one sentence long and a child can start right away. Still it practices real reasoning. You work toward each other from both ends of a gap and have to count whether the way in between is long enough. Along the way the number sequence up to a hundred is rehearsed without ever being tested.
Common pitfalls
The most common mistake is the step across a corner. Diagonal does not count as neighboring here, even if two fields touch at the corner. The second pitfall is guessing. Filling a gap at random gets you stuck further along - better to count first how many fields are really needed.
How to practice at home
Draw a 4 by 4 grid on squared paper and enter your own path from 1 to 16. Then rub out a few numbers and hand the puzzle to somebody else. Building one yourself is the fastest way to understand the rule.
Related: Neighboring Numbers, Math Maze