Sharp glass edges are heated to make them smooth. Because on heating glass melts and the surface of the liquid tends to take the rounded shape at the edges which has minimum surface area. This is called fire polishing of glass.

When a liquid flows over a fixed surface, the layer of molecules in the immediate contact of surface is stationary. The velocity of the upper layers increases as the distance of layers from the fixed layer increases.

This type of flow in which there is a regular gradation of velocity in passing from one layer to the next is called laminar flow.

In laminar flow, the velocity of molecules is not same in all the layers because every layer offers some resistance or friction to the layer immediately below it.

It is possible to change a gas into a liquid or a liquid into a gas by a process such that there is always a single phase present.

e.g., In figure given above, we can move from $A$ to vertically by increasing the temperature, then we can reach the point by compressing the gas at constant temperature along the isotherm (isotherm at $31.1^{\circ} \mathrm{C}$ ). The pressure will increase. Now, we can move vertically down towards $D$ by lowering the temperature. As soon as we cross the point H on the critical isotherm, we get liquid. Thus, at no stage during the process, we can through two-phase region. It process is carried out at the critical temperature, substance always remains in one phase. This is called continuity of state between the gaseous and the liquid state.