A tornado forms when a cloud base lowers and a funnel cloud forms. While the funnel cloud is descending, winds build near the ground and kick up dust and debris. This kicking up of debris causes damage and destroys property.
Some tornadoes have a visible funnel that extends down to the ground, but other tornadoes may not form a funnel. The air feeding into the storm may be warm and dry, so it doesn't condense any water vapor into a visible funnel.
Tornadoes can appear quiet before they strike, but this may be misleading. In fact, a tornado could form at the surface before radar detects rotating winds.
The eye of the tornado appears calm and clear because some of the rising air is forced to converge towards its center. This convergent air is deflected by the coriolis force and doesn't reach the center.
If a tornado is headed your way, find a shelter immediately and stay away from windows and outside walls. If possible, go to a basement or an interior room on the lowest floor of your home without windows. For added protection, get under something sturdy such as a table or workbench.