Had James Tarkowski not intercepted the ball, Luis Diaz would have been called offside.
But because the Everton defender did, his defensive instincts kicking in, Diaz was onside.
This moment in Wednesday's Merseyside derby led to Liverpool's winning goal, scored by Diogo Jota.
Everton were not happy. So what are the offside laws and are they fair?
What happened?
