First look only at the two halves next to the gap.
The tile you need must have exactly that number of dots on the left and that one on the right.
A domino tile may be turned around - so check both ways before ruling a tile out.
Example
For parents
What does this exercise practice?
The domino chain practices careful comparison and the insight that a tile has to satisfy two conditions at once.
Common pitfalls
Often only the left side gets checked. Both neighbors have to match, otherwise the chain breaks on the other side.
How to practice at home
Build a chain with a real set of dominoes, take one tile out and let someone find it again.