Stress balls are ancient by toy-industry standards. NeeDoh shows what happens when an old need collides with social media, collectibility and a particularly stressed-out culture. Somewhere along the way, the humble stress ball became cool. The current star is NeeDoh, a line of brightly coloured squishy toys from Schylling. NeeDoh itself isn’t even new. It launched in 2017. Then TikTok got hold of it. By February, Schylling says it had burned through six months of inventory in six weeks. Sales...