What it shows:
A liquid in a number of different shaped communicating vessels will find the same level in each.
How it works:
The pressure at the bottom of the fluid depends upon the depth of the fluid and not on the shape of the container. The apparatus consists of a group of glass flasks of assorted shape (see figure 1) linked at their base by a communal reservoir. With the pressure being dependent on the depth of liquid only, an equilibrium situation must have the surface level in each vase equal.
figure 1 Pascal Vases
Setting it up:
The flask's dimensions (20cm in length by 10cm height) make it a good size to sit in front of a light box. If colored water is added, the levels will show up well.
It is probably a good idea for the lecturer to add the water so the leveling can be seen.