Correct definition of surface tension

[mcfaker]

Hi,

I have a question, because I am confused with the definition of surface tension.
In my book it is defined as follows:
"Surface tension is the energy required to increase the surface area of a liquid
by a unit amount"

What do they mean with increasing the surface area, how do you do that? If I take a glass with water it will look like this:

I don't think there is any possible way to increase A. Can someone please help me out?

Thanks in advance!

 

[few]

(A) in your diagram is a flat plane basically. Its surface area can be increased by pulling it upward or downward in the middle for example.

The definition in your book is a little confusing because it is from a physics perspective and that is how surface tension is measured; force/length (N/m)