Not actually how volume works

When I was a kid, I thought that if you had two liquids, of volume A and B, and you mixed them together, the result would have volume A+B if they were the same liquid, but volume max(A,B) if they were different.

It was clear to me that this didn't make sense, but it was what I had concluded from observation. I was really confused though because I didn't understand how to resolve the paradoxes this rule plainly led to. I think finally I asked my parents about it, and I think they took a while to understand what I was asking, because the question was so strange.

Anyway they did at least manage to finally understand enough to explain that that's not how volume of liquids works...