For this activity you need some dry ready mixed concrete. It comes in bags from DIY shops.

Take a small handful of the concrete and look at it. See that it has grey cement, yellow sand and some stones in.

Place it on a dish and add TINY drops of water until you can stir the concrete into a stiff paste.

Now make any shape you want and stand it on the window ledge to dry.

Go back two days later and see what the concrete is like.