急求一英语脑筋急转弯的答案!并说明理由!

这道题的全文是 “What Runs But Never Walks,Has A Mouth But Never Speaks,Has A Head But Never Weeps,And Has A Bed But Never Sleeps?”
翻译成中文是 什么只跑不走,有口却不语,有头却不哭,有床却不睡.
答案是river.英文中river runs 没有river walks,有河口(mouth)但是不会说话,有源头（head）但不会哭泣,有河床（bed）但是不睡