I had house #1 paid for @40 yrs old-1990 value-275k.
I qualified for a mortgage and bought a 2500sf condo for 230k. My wife said 2 years later we need more room. I again qualified for a second mortgage but failed to get asking price for condo so I rented it for 2k a month.Bought current house 2003 289k.
So in 2003, House 1 rent pays mortgage on condo,and rent from condo pays mortgage on principal residence. All of a sudden I control 3-300k properties, and have no payment now i am underwater on 2 of 3.