为什么用switch语句没有效果？

score =prompt("请输入你的成绩");
    switch(score)
    {
        case score>=90:
            document.write("你很棒!");
            break;
        case score>=75&&score<90:
             document.write("不错吆!");
             break;
        case score>=60&&score<75:
            document.write("要加油!");
            break;
        case score<60:
            document.write("要努力了");
            break;
    }