Water is heated from below but not from top. Why?

Explanation

When water is heated from below, its density decreases, it becomes light and rises up. Cooler water from above comes down and takes the position vacated by hot water. So convential currents are set up and the whole water gets heated.
When water is heated from top, it will conduct very small amount of heat to the bottom part of water because water is a poor conductor of heat. hence water will not be heated properly.