Why does our hand or foot suddenly “fall asleep”?

When we feel a tingling feeling in our hand or foot, we say that our hand or foot “has fallen asleep.” That usually happens after we have crossed our legs for a while or after we have placed a heavy burden on our arm. We can understand why this happens by comparing the crossed leg or the burden-laden arm to a curved pipe. Water running through a curved pipe cannot flow freely, and can only drip through slowly. Similarly, the blood in our crossed leg or burden-laden arm flows more slowly than normal. Since it cannot flow at its normal speed, it is unable to carry out its various important functions. One of those functions is to gather all the waste products that have accumulated in the body. If this waste material is not removed from the body, it disrupts the nerves, which carry messages from our foot or hand to our brain, and, as a result, our brain does not know exactly what is going on with our hand or foot. When we shake our foot or stop the pressure that was slowing down the blood circulation in our hand, the brain receives a larger number of messages than usual from our hand or foot. These messages cause the feeling of “pins and needles” that we experience.

When was the first electric refrigerator invented?

Over 2,000 years ago, the Chinese were already chopping ice from glaciers and frozen lakes and storing it for use during the summer months. And 1,500 years ago, the Egyptians and the Indians created ice by leaving water in a clay jar outside their domiciles on cold nights. Some 300 years ago, British servants would collect ice during the winter, wrap it in rags and keep it in special places until summer.

A refrigerator that runs on electricity, and not on ice provided by external sources, was only invented 100 years ago (in 1911) by General Electric. The first refrigerator was immense, and was activated by a motor the size of an entire room! It took years of refining until the refrigerator became a common appliance, found in every household.

How does ice make things cold?

If your finger touches a piece of ice, your finger will feel cold. At that moment, it might seem to you that the cold is coming out if the ice and entering your finger, but in truth, it is just the opposite – the heat that is coming out of our finger is being transferred to the ice!

The proof that this is what is happening is the fact that the heat coming out of your finger melted a small quantity of ice and your finger is wet. When you place a cube of ice in a glass of juice, the ice will absorb some of the heat in the juice and will slowly melt. The piece of ice gradually shrinks while the juice gradually becomes colder.

If you want to slow down the rate at which a chunk of ice will melt, just cover it with a blanket. That idea might sound a little silly, but it really works. When the chunk of ice is totally exposed to the air, hot air hits it from every possible direction. The ice absorbs the hot air and starts to melt. However, if we cover the ice with a blanket, the blanket will keep out the hot air and the ice will melt much more slowly.

Why do the birds that sit on electric wires not get electrocuted?

Electric cables are extremely dangerous because they generally carry a very powerful electric current that can carbonize anything that comes in contact with it. That is why you must never touch an electric cable that has been torn by a storm and which is lying on the ground. Nevertheless, we often see birds sitting on electric cables without suffering any harm. How do they manage not to get hurt?

An electric current can move from a high-tension wire to the ground or to another high-tension wire by means of a conductor – a substance that enables electric current to pass through it. If a bird simultaneously touches an electric cable and a tree, for example, it will certainly be electrocuted because, in that case, the bird’s body becomes a conductor linking the electric cable to the tree and, from there, to the ground. Similarly, if a bird simultaneously touches two electric wires, it will be electrocuted. However, generally speaking, a bird touches only one electric wire at a time. Thus, its body does not serve as a conductor and the bird is not harmed.


Question of the month:

This month’s question was submitted by Alex of Brooklyn, NY

Dear Professor,

How many eyes do spiders have?

Dear Alex,

Most spiders have eight eyes, but only two of them, the central ones, are capable of creating images of the spider’s surroundings. The side (lateral) eyes are simpler and cannot transmit a clear image, but they can detect movement around them and can spot prey passing by in the area.