How to Break It
That should be enough to get you started, but first let’s break this Makefile in a particular way so you can see what happens. Take the line rm -f ex1 and remove the indent (move it all the way left) so you can see what happens. Rerun make clean, and you should get something like this:
$ make clean Makefile:4: *** missing separator. Stop.
Always remember to indent, and if you get weird errors like this, double check that you’re consistently using tab characters because some make variants are very picky.