Consider the following statement I found in a physics book :
"The net flux through the surface is proportional to the net number of lines leaving the surface, where the net number means the number of lines leaving the surface minus the number of lines entering the surface. If more lines are leaving than entering, the net flux is positive. If more lines are entering than leaving, the net flux is negative."
I don't understand how can an electric field line enter a closed surface and does not leave it... These lines are infinite right ? So if a line is entering then it must also leave the surface. Can someone illustrate me the situation they are talking about ?