One of my friends teaches mathematics in primary school. He was asked a question that Why $(-1)\cdot (-1)=1$ ...
At higher level we can answer this question saying that it is a definition and we want different things such as associative and distributive law to hold but how we convince a primary level student.

I don't want any proof as I know very well how to prove..I want an intuition for a primary level student. I know how to prove it.