How to access one component's state from another component

How do I access one component's state in another component? Below is my code and I'm trying to access the state of component a in component b.

var a = React.createClass({
    getInitialState: function () {
        return {
            first: "1"
        };
    },

    render: function () {
        // Render HTML here.
    }  
});

var b = React.createClass({
    getInitialState: function () {
        return {
            second: a.state.first
        };
    },

    render: function () {
        // Render HTML here.
    }  
});

But I'm not getting anything.