Stroop Effect Pack

Trick your card-sharp friends - red spade and clubs, black hearts and diamonds.

This is a pack of cards with the colours of the suits reversed: the clubs & spades are red, while the hearts & diamonds are black.
The aim of the pack is to disconcert the seasoned play by making an involuntary pause in play. This is called the Stroop Effect - the delay in reaction time between congruent and incongruent stimuli - after which the pack is named.