What exactly did we tell Tk with the previous command? When the button was
created, a new command was made with the same name as the button. This
command can be used to change things of the button after it has been
created. .b configure tells Tk we want to change some option of the
button named .b. The -command option is the option that tells
Tk what to do when that particular button is pressed. Other options can
change the text in the button, change the background color, change the
dimensions and a lot more.