From The Guardian:
Consider the squishy. The point of the squishy, a palm-sized mass of polyurethane in the shape of a fruit or a croissant or a unicorn cat, occasionally scented with strawberry, is to squish. The point of the squishy is to be held in the hand of a person with energy that needs redirecting and for them to direct it into the soft heart of the squishy, to squeeze it into almost nothing in their palm, only for it to reinflate again, asking for more. In 1988 a TV writer called Alex Carswell threw a pen at a photo of his mother after a stressful phone call with his boss. It gave him an idea.
It was the "Age of Stress" – the Daily Mirror (among other newspapers) had identified it as "a killer" – and so the perfect time for Carswell to launch his "stress ball". By the 1990s it had evolved from something squishy designed to be thrown into something squishy designed to be squeezed, and to be squeezed mainly by kids, who collected them in small scented families in their rucksacks. In a 2015 study of patients undergoing varicose vein surgery, those that handled stress balls reported feeling "less anxious".
"When you're stressed, your body tightens up," says Dr Kathleen Hall, founder of the Stress Institute, explaining why throwing or squeezing something feels good, "so a physical release helps to let go of some of that energy." Carswell was not the first person to link a calming of the mind to a busy-ness of a hand – in 206 BC, the Han dynasty in China trained to stay mentally focused during combat by squeezing walnuts. The croissant squishy comes from an ancient place.