The Strangest Everyday Substances: Solid or Liquid?

All of us would say that we can tell a solid from a liquid. Water is a liquid and a cup is a solid. A liquid takes the shape of its container. So, are sand and lava liquids? And what about yoghurt or ketchup, which only start to flow when we shake them up? The classical divisions of the states of matter, which we think we know so well, are not always so simple.  

Javier Yanes