HEY THERE, AGAIN,

I d like to know how to create an on-mouse comment to my blog posts.

Like when I write:

I saw a huge papaya at the local store this morning.

then papaya is double underlined or highlighted or whatever and when I move my mouse over it, it ll pop up a box saying:

Papaya - a type of fruit, widely used in Thailand to make wonderful dishes, such as Som Tum Thai.... yum

How is this possible ?

thanks