Lowdown

Email your question(s) to flipside@flipside.org.uk or send on a postcard or letter, marked 'The Lowdown', to the address here.

Why do we have that coloured bit in our eye and why do they have different colours in different people? Also my eyes look different colours in the light. Why is this?
SW-W, by email

The coloured bit is called the iris and is a bundle of tiny muscles that let the black pupil at the centre shrink and grow so that you are not blinded by bright lights. Curiously, there is only one coloured substance sitting in the eye. That melanin is only ever brown - it's the same stuff that makes skin look brown.

People with blue or green eyes only have melanin across the back of the iris. The colour comes from the way light bounces around the grey muscle fibres - the closer they are together, the more blue they will usually look, rather than green. When the light changes from being white, the eye will seem to pick up some of that colour so your eyes can end up matching what you wear.

When melanin is on the front of the iris, your eyes just look brown. Some people have very little melanin even on the back of the iris, so some red light reflected from blood vessels at the back in the retina shines through, producing violet eyes.

If you stopped milking a cow, would its udders explode?
TS by btconnect.com

It’s not a good idea to just stop milking a cow. The amount of milk a cow produces depends on how much gets taken out at each milking. If the farmer takes less, then the cow will produce less milk to replace it. Take a lot, and a lot more will get made unless the cow is beginning to dry up. That happens just before it gives birth to another calf. If you just stopped milking then the udders would get very full – and you’d have a really uncomfortable and irritable cow for a few days until its body re-absorbed the extra milk. There is also a good chance of the cow picking up an infection if it’s not milked. But there’s no danger of exploding cows.

Why are some animals immune to snake bites?
LS by gmail

It's not just the snake venom that kills people - sometimes their own bodies react so badly to the poison that they turn what ought to be a mild sting into something deadly. The culprits are mast cells. These are special cells in the body that stop parasites from spreading. But, faced with the toxins in a snake bite, the cells can run amok and produce their own deadly poisons.

But, in some breeds of mice, the mast cells work the other way. It takes ten times as much poison to kill a mouse with the right mast cells than a creature that does not have them. The same cells work against the venom in bee stings. But only some creatures have the right cells - so most don't try to mess with snakes.

Why does the water stay in lakes: why doesn’t it all just drain out through the soil at the bottom?
PW, Cardiff

If the water stays in a lake, it’s because there isn’t anywhere else for it to go. The ground underneath the lake is already saturated with water. When there is a really bad drought and the lake dries up, it can take longer than expected to refill because the ground beneath the lake may dry out as well. The waters can’t begin to rise in the lake until the ground is saturated again.

Does water flow down plugholes in different directions in different hemispheres?
ER by hotmail.com

The water should spin counterclockwise as it flows out the bottom of bathtub here in the UK. And it should do the opposite in Australia. But it’s only going to happen if nothing else upsets the flow. The reality is that the Earth’s spin – which is meant to cause the effect – is so weak that you will never see it drive the water one way or the other. How the water spins when it empties is far more likely to come from how you pull out the plug, how the water pours into the tub and any slight quirks in the shape of the tub.