When you can take a shit at your true friends house, without worrying about skid marks, the stench or even having to flush.
ME: "Mum, Greg has officially been moved into my 'true friend' zone.
MUM: " What?"
ME : I took a dump at his house and didn't flush. Didn't even feel guilty.
MUM: "That's how me and your Dad got through the friend zone"
ME : "..."