Convection In Water

When the water at the bottom of the flask is heated, it expands. The expanded water is less dense than the surrounding water and therefore starts to rise. In doing so, the cooler regions of the water in the upper part of the flask, being denser, sink. This movement of the liquid due to a difference in its density sets up a convection current.

Convection currents occur only in fluids such as liquids and
gases but not in solids.

This is because convection involves the bulk movement of the fluids which carry thermal energy with them. For solids, the thermal energy is transferred from one particle to another through vibrations, without any bulk movement of the particles themselves.