Designed for 10 - 15 year olds (kids get all the fun, grumble grumble), this Big Eye Cruiser bike can be adapted whenever the teen has a growth spurt, with the frame stretching horizontally.

The frame, with the comfortable-looking seat, can be angled to suit the user's height, and even sex. It's a concept by Claudia Baer, Anna Wiesinger, and Marlene Klausner, but actually very practical—unlike a lot of other concepts we feature on here. [Yanko Design]