A Crowded Coop - Portal panneau m&eacute;tal Paradox Know your paradoxes! Wall sign from the Portal series. Put Aperture Science Laboratories' rules onto your wall! The Portal 2 Paradox Tin Wall Sign comes from Valve's Portal video game series. If you're fond of the turrets in Portal, you're probably going to want this Portal 2 Paradox Tin Wall Sign. It features a turret with recommendations on what to do in the event of seeing rogue AI: 'Stand Still. Remain Calm. Scream.' The sign measures 11-inches wide x 5 1/2-inches tall and makes great bedroom decoration for the Portal 2 fan!