JavaScript prototype behaviour

gistfile1.txt

function Thing() {}
//undefined
var t = new Thing();
//undefined
t instanceof Thing
//true
t instanceof Object
//true
Thing.prototype.__proto__
//Object {}
t.__proto__
//Thing {}
Thing.prototype
//Thing {}
t.__proto__ = String.prototype;
//String {length: 0, [[PrimitiveValue]]: ""}
t instanceof Thing
//false
t instanceof String
//true