Question

In a fish tank, a small heater in one corner makes all the water warm
How?

Answer 1

The small heater has warmness but the water has molecules so mix those together and that gives you an warmed up water tank

Answer 2

As the water molecules near the heater warm, they diffuse. This means that they move from an area of high concentration to an area of power concentration. As this occurs, the spread of warm molecules evens out and causes an average temperature in the water.