First of all alt is an attribute, not called as a tag.
It's used to show alternate text in place of an image displayed through <img> tag, when the browser is set not to display images.
If you are trying to display a tootip, you need to use the title attribute. (IE erroneously show a tooltip, for an alt attribute)
The Dream is not what you see in sleep; Dream is the thing which doesn't let you sleep. --(Dr. APJ. Abdul Kalam)