Rose: No 1 mom and 1 mom equals to 2 kids that means 4...
Ray: she is so weird...

Gaster: -_- it's 2...

Rose: Then it's true 1 +1 = 3 heh
Ray: Stop with the jokes...

Gaster: quite simple actually. Magic.
Although, trying to explain the entire prosses of how it is possible often causes me stress. A lot of it. And you should remember what happens when I get overly stressed.