Weird Fact: It is possible to walk on custard, Jesus-style.

My childhood memory of custard is of thick, gelatinous curd that you could cut with a knife. However, most people have a more realistic impression of custard: runny liquid stuff that you couldn’t possibly walk on – or could you?

As the following video clip from Brainiac shows, you can indeed “run around on a sea of custard like a demented lemming”.

But WHY?

Normal liquids (or “Newtonian fluids”) have a constant viscosity – their flow only changes with different temperatures. For instance, water is a solid at  zero degrees Celsius and a gas at 100 Celsius. It’s a constant runny liquid anywhere in between, demonstrated by the fact that it takes on the shape of any container it’s poured into.

However, custard is a non-Newtonian fluid. It changes its viscosity under stress, so if you apply sudden force or pressure, it suddenly becomes thicker and behaves like a solid. Other non-Newtonian fluids can go the opposite way – and become runnier under pressure. When you take the stress away, they all return to a common liquid state.

Tomato ketchup is another example of a non-Newtonian fluid. The cheeky sauce may refuse to come out the bottle when you simply tilt it upside down and wait for gravity to do its thing. But when you give the bottle a hard slap, you’re actually applying a sudden force which reduces the viscosity and prompts the ketchup to be runnier, easing it out the bottle faster.

Now to the proof – you can walk on custard – in spectacular fashion:

[youtube=http://www.youtube.com/watch?v=4yqjAmSbgf0;&w=450;&h=300]