Three amateur pilots take on a team of stunt fans in a challenge to build a machine that will allow Richard to feed his dog without leaving his desk. With a place in the semi-finals at stake, the rival teams let their imaginations run wild - and even spy on each other's work in a bid to get ahead. Chain reaction machine guru Zach Umperovitch is the expert judge, while Shini Somara explains the science behind the eccentric contraptions.

The Fieldhouse Brothers, from Somerset, take on the Bueno Brothers, originally from Spain, in a closely fought first semi-final to build jaw-dropping chain reaction machines that will allow Richard to dust his ornaments without leaving bed. With a place in the final at stake and a challenge that requires delicacy and creativity, the contraptions need to be extra special. Chain reaction machine guru Zach Umperovitch is the expert judge and Shini Somara fills in the science behind the teams' eccentric contraptions.