Thermal expansion: real and apparent expansion of liquids

A round bottomed flask is filled with water up to the level $L_1$.

When the flask filled with water is heated instead of the water level rising, it initially drops to $L_2$. This is because before the water starts expanding, the glass of the flask expands which cause the water level to drop for a little while.

Once the water starts to expand the water level rises to $L_3$.

To an observer it might appear that that the water level increased from $L_1$ to $L_3.$ However, in reality the the water expanded from $L_2$ to $L_3$.

Therefore we can say that,

Apparent Expansion of Water: $L_1$ to $L_3$

Real Expansion of Water: $L_2$ to $L_3$