67 is funny cuz um it has no meaning. this makes it funny cuz um we expect things to have meaning so breaking that norm is funny. also saying 6-7 makes us feel like we are part of a social ingroup of people that think "dumb" and "meaningless" things are funny. also, the hand gesture and rythmic and um song like tone make 6-7 fun to say and makes it funnier. in summary, the meaninglessness of 6-7 makes it funny, not less funny.