$A=\{1,2,3,4,5\}$
$B=\emptyset$
How to explain to a learner from 6th grade the result for $A \cap B$ ?
For a 6th grade student, it could be difficult to understand why the result is $\emptyset$. Most of them might think like this: ...well, the result is what both $A$ and $B$ have, but does the $A$ set contain the $\emptyset$ ?
It will be great to explain with some day-by-day examples, examples from real life and so on.
Assuming he already knows what an empty set is, it should not be too hard.
Usually, a good way to represent sets is bags with things in them, and an empty set is just a bag with nothing in it.
Now, you can represent an intersection of sets with picking common things. So if one bag has eggs, flour and butter and the other has butter, milk and bread, the intersection of them is a bag which has things that are in both bags, so it has milk.
An intersection of a bag with milk and eggs and a bag with bread and butter is then an empty bag, because no object is in both sets.
Now in your case, one bag is empty, so the intersection with this bag is empty, because nothing can go into the intersection because nothing is in the empty bag.