Why do the Muslims 'kiss" a particular stone at Kaaba, in Mecca, during hajj or other times?

What's the scientific reasons, would any Muslim scientist explain? Isn't kissing the same stone by millions of Muslims from hundreds of countries mean an unhygienic action? What a lifeless stone could be 'something' to the Muslims when they're against worshiping dolls or sculptures etc? Isn't it similar to the Hindus worshiping a stone, called "Shiva's "? (simply nonsense!)