Her smile is natural.

In mathematics, an empty set refers to a set that does not contain any elements.

This battleship has enhanced antiaircraft capabilities.

This area has a strong air defense system.

His life followed a path of downfall.

His company fell into ruin due to the economic crisis.

He tumbled down the stairs.

He experienced an economic fall.

He is a true visionary, always coming up with new ideas.

He used cement to close the hole.