How can an object in Java be an instance of two different classes?

I’m working in the Rhino JavaScript engine (implemented in Java) in a context where a Java object has been shared into my JavaScript code’s runtime. It seems the object is somehow simultaneously an instance of two different classes.

When I use instanceof to try and find out what kind of object it is, it passes for two different classes:

foreignObject instanceof Packages.java.lang.String;  // true
foreignObject instanceof GlideElement;               // true

How could this have been implemented? Is there a pure Java way to get an object to pass both these instanceof tests? Is there some other kind of magic going on here specific to the Rhino JavaScript engine?