state why a siphon need to be full before starting it

Question

Why should a siphon be full of a liquid before starting it

Answer 1

It should be full because after this the liquid will continue to run out below the level of the liquid in the tank

Answer 2

To remove the air pressure that might have been trapped inside.

Answer 3

To lower the air and atmospheric pressure in it

Answer 4

In order to create a pressure difference 

Answer 5

To ensure that there is no air inside and also to create pressure difference