virtual function

Question

class super {
public:
virtual~super(){cout<<1;}
};
class sub: public super {
public:
~sub(){cout << 2;}
};
int main(){
super*ptr = new sub
delete ptr;
}

// outputs 21

Why does the 1 get outputted?
I believe that the pointer points to the sub class's sub function, which prints out 2.
But I don't understand what allows the base class's super function to be expressed...

Is there something I should know about virtual functions?