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If you pump air through sand, it turns into a fluid. https://thekidshouldseethis.com/post/royal-institution-fluid-sand-video https://thekidshouldseethis.com/post/royal-institution-fluid... An important example application is in getting grain to flow smoothly out of a grain silo. Some substances are "non-Newtonian fluids" meaning that they are liquid normally but turn solid under pressure. Famously, a mixture of cornstarch and water does this, which kids can have endless fun with: https://www.youtube.com/watch?v=Fnd-2jetT1w https://www.youtube.com/watch?v=Fnd-2jetT1w An example application is bulletproof vests which allow their wearers to move normally but will stop a bullet.