Fill the first two numbers of a series ‘1’ and ‘2’ as shown.
Select both cells by clicking on the top cell and dragging the mouse down.
Then click on the fill handle and drag down to fill the series.
You can use this to automatically number rows or columns. It also works with calendar dates, and even days of the week.
The filldown icon allows you to change the type of fill. For example if you change to 'copy cells', the fill will change to '121212'.