Why are manhole covers round? Because they copied my shape. No Manhole covers are quite heavy. So, round-shaped covers can be rolled easily from place to place. Also, round covers can simply slide and fit into the hole. But, to fit square or triangular manhole covers properly. They must be precisely aligned with the edges and angles of the manhole. Now, the manhole has a small lip around the rim of the hole. This lip makes the opening smaller than its cover and prevents the cover from falling into the hole. But, a square or rectangular cover can still fall into the hole, if inserted diagonally. Why? Because their sides are always smaller than their diagonals. A round cover won’t fall into the hole. Because no matter how you place it, the cover is always wider than the opening. Topic: Respiration Why is it not good to sleep under a tree at night? Hey. You look so tired. Why don’t you take rest here? No. Please don’t sleep under that tree!! It is quite harmful. See, you are not able to breathe properly, right? Do you know why? Wait, I will tell you. This is because during the day, in the presence of sunlight, plants take in carbon dioxide. Release oxygen in the process of photosynthesis. However, they even respire simultaneously. In this process, plants take in oxygen and release carbon dioxide. But the production of oxygen is more than the production of carbon dioxide. Hence, if we sleep under a tree during daytime, we may get a good amount of oxygen. Thus giving us a nice sleep. However, at night, plants do not perform photosynthesis due to the absence of sunlight. But respiration still goes on. Hence, as compared to oxygen, the proportion of carbon dioxide around the trees is more. Thus, if we sleep under a tree at night, we may feel suffocated due to lack of oxygen. Moreover, inhalation of excess carbon dioxide is harmful to human beings. Therefore, it is not good to sleep under a tree at night. Topic: Paper cut. Why do paper cuts hurt so much? It is because paper hates us. No. We use our fingers and hands to sense our surroundings. Hence, they have more number of nociceptors than any other parts of our body. Nociceptors are pain receptors that respond to change in pressure, temperature, etc. Now, the edges of the paper are not smooth but are rough or jagged. They are like a saw. Absolutely. Hence, sometimes when we rub our hands or especially fingertips against the edges. It cuts themlike a saw, activating the nociceptors. Now, as there are more nociceptors on our hands, we feel more. In addition to this The paper cut might also have fragments of paper containing chemicals. Which may irritate the skin and thus increase the pain. How do igloos keep you warm? I’m a little busy. So, I’ll tell you about it later. Alright. I’ll explain. Heat is a form of energy present in our body. When the temperatures around us are low, we begin to lose heat. The more we lose heat, the colder we feel. Now, although igloos are made up of blocks of compressed snow, they keep us warm. Stop bluffing. How’s that possible? Compressed snow has air trapped in it and air is a bad conductor of heat. Hence, the heat given off by our body does not get lost from the igloo. It stays in the igloo, thus keeping us warm. Besides this, we know that warm air rises up and cold air sinks to the bottom. Hence, During night, Eskimos, that is, people living in igloos sleep at the uppermost part of the igloo to stay warm. Why smoking makes people look old? So that we look more experienced and intelligent. No. Smoking makes people look old because things like cigarettes contain nicotine. Nicotine is a vasoconstrictor which means it narrows our blood vessels. Restricting the blood supply. When the blood supply is restricted, our cells don’t get the required oxygen and nutrients. As a result, our skin becomes dry and discoloured. In addition to this, the collagen and elastin fibers which keep our skin firm and elastic. Begin to break down much faster, leading to saggy and wrinkled skin. Also, when we smoke, we purse our lips and squint our eyes. Regularly repeating these actions may also cause wrinkles on our lips and around our eyes. In this way, smoking makes our skin dry, saggy and wrinkled, thus making us look old.