Generic method to type casting

I'm trying to write generic method to cast types. I want write something like Cast.To<Type>(variable) instead of (Type) variable. My wrong version of this method:

public class Cast
{
    public static T To<T>(object o)
    {
        return (T) o;
    }
}

And this is simple test:

public class A
{
    public static explicit operator B(A a)
    {
        return new B();
    }
}

public class B
{
}

A a = new A();
B b = Cast.To<B>(a);

As you guessed, this code will fail with InvalidCastException.

Is this code fail because virtual machine doesn't know how to cast variable of type object to type B at run-time? But exception message says: "unable to cast object of type A to type B". So CLR knows about real type of variable o, why it cannot perform casting?

And here is main question: how should I rewrite method T To<T>(object o) to fix this problem?