BRAINDUMP
Amazing answers to your curious questions
Several species of sharks are known to live in freshwater environments, but whether they should be considered true freshwater fish is debatable. Probably the most mysterious are the river sharks of the genus Glyphis. Fewer than ten species have been identified in watercourses around southeast Asia and Australia, with some still waiting to be officially classified, and all are extremely rare.
Much better known is the bull shark. This worldwide species enters warm water estuaries from the ocean and then swims into fresher water upstream. If you see an adult bull shark from a kayak, you might wish you had a bigger boat, as they are large and aggressive predators and have been known to attack humans. A notable characteristic of bull sharks living in freshwater habitats is that they excrete a large amount of urine. Sharks that evolved in the ocean have a great deal of salt naturally present in their bodies to prevent them from losing water to the sea through osmosis. In the non-saline water of rivers and lakes, bull sharks have the opposite problem: they would swell up like a water balloon without a way to get rid of the excess freshwater that their bodies absorb. As a result, they urinate quite a lot!
WHY DOES THE BUZZ OF A FLY AND A WASP DIFFER?
The buzzing sound of a fly or wasp is created by the vibration of the insect’s wings. The human ear interprets frequency as pitch. The higher or lower the frequency, the higher or lower the pitch. Fourwinged insects like wasps and bees flap their wings at a slower frequency than twowinged flies, resulting in a deeper buzz.
CAN SHALE GAS CAUSE TAPS TO CATCH FIRE?
To extract methane gas within shale, mining companies pump water down a borehole to force cracks to widen. This can drive the methane up into shallower rocks containing groundwater. When this water is extracted it retains some dissolved methane. In extreme cases, you can hold a lit match to the stream of water coming out of a tap and the methane will set alight.
WHY DO PAPER CUTS HURT SO MUCH?
Paper can cut your skin as it’s incredibly thin, and if you look at it under a high-powered microscope, it has serrated edges. Critically, though, a sheet of loose paper is far too soft and flexible to exert enough pressure to pierce the skin, hence why they are not a more frequent occurrence. However, if the paper is fixed in place – maybe by being sandwiched within a pack of paper – a sheet can become stiff enough to attain skin-cutting pressure. Paper cuts are so painful once inflicted as they stimulate a large number of pain receptors – nociceptors send nerve signals to the spinal cord and brain – in a very small area due to the razor-type incision
