Also fun exercise: Go through Euclid's Elements (i.e. http://aleph0.clarku.edu/~djoyce/elements/bookI/bookI.html http://aleph0.clarku.edu/~djoyce/elements/book

Also fun exercise: Go through Euclid's Elements (i.e. http://aleph0.clarku.edu/~djoyce/elements/bookI/bookI.html http://aleph0.clarku.edu/~djoyce/elements/bookI/bookI.html) Read the definitions and then prove all the postulates yourself, in order.