為什么用switch語句沒有效果？

score =prompt("請輸入你的成績");
??? switch(score)
??? {
??????? case score>=90:
??????????? document.write("你很棒!");
??????????? break;
??????? case score>=75&&score<90:
???????????? document.write("不錯吆!");
???????????? break;
??????? case score>=60&&score<75:
??????????? document.write("要加油!");
??????????? break;
??????? case score<60:
??????????? document.write("要努力了");
??????????? break;
??? }