What should be your answer when a $5 \space th$ standard student ask you to explain why $2+3=5?$ What should be your answer when a $5 \space th$ standard student ask you to explain why $2+3=5"?$
For me this is really difficult to explain to the little kid without set theoretic construction. So, I want to know what should be a convenient answer according to you ?
I know this is slightly off topic question but this is a correct place to ask.

The optimistic interpretation is that this student is starting to understand that mathematical facts are all true for actual reasons (as opposed to "my book told me this rule"), which is an attitude we absolutely want to support and encourage! It's just that this student is exploring this space-of-"why" in a context where there's not as much going on as they suspect.
I guess the best thing to do is to point out the difference between mathematical objects and properties being defined and being deduced. "Why is every number ending in 0 a multiple of 5?" is a great "why" question that's about deductions and implications. But the answer to "Why is $2+3=5$?" is just definitions: each natural number has a name (like $2$ or $5$), and addition has a definition, and we know in advance that $2+3$ is some natural number that has a name, and the name of that natural number just happens to be $5$.
If the question "Why is $2+3=5$?" came in a context like "We know that $5$ is the number after $4$; ...", then it's more about deduced mathematical properties. But in and of itself, I don't think there's anything to say ... other than congratulating the student for wanting to know why things are true rather than just accept them, and helping them understand that there are different kinds of reasons why different kinds of math facts are true.
