Connect the Dots, Brain Test

In this Briquinex brain exercise, you are at the school and the teacher, draws on the table 9 dots that, are arranged in a three by three square (see photo). He asks you, to draw it also on your paper and to connect each of the nine dots using only four straight lines and without lifting your pen from the paper. How will you do it? how about connecting the 9 dots using only 3 lines?