What do the words hydrant, hydroplane, hydrology, and hydrotropic have in common?

The words hydrant, hydroplane, hydrology, and hydrotropic all have a common root word "hydro," which is derived from the ancient Greek word for water, "hydor."

The words hydrant, hydroplane, hydrology, and hydrotropic have the common root word "hydro," which is derived from the Greek word for water, "hydor."