I left my mountain bike with my dad (along with Tom's bike helmet) - and the stipulation that he doesn't ride without the helmet.

the last time he rode a bike, this happened:

Admittedly...the problem was Dad doing something that shouldn't be done on a bike (the seat was too hard so he put a pillow on it...when the pillow slipped, so did he).

the bike I left has really fat stable tires, is an adult bike (unlike the one he fell off) and has a padded seat.