why can you usually not tickle yourself? your brain predicts the touch you are about to make and turns down the expected sensation

why can you see through a microwave door mesh? its holes are tiny compared with microwave waves but huge compared with light waves. light gets through, while the mesh reflects microwaves

why do your ears pop on a plane? pressure changes around your eardrum. when the eustachian tube opens, it equalizes the pressure

why do jeans have metal rivets? they reinforce the points that pull hardest, especially pocket corners and seams

why do we shiver when cold? your muscles start rapid involuntary contractions. that activity produces extra heat

why do f and j keys have bumps? they are touch markers. your index fingers find f and j, then the rest of the home row follows

why do we get goosebumps? tiny muscles at each hair follicle pull the hairs upright. the skin dimples into goosebumps

why do tires have little bars in the grooves? they are wear bars. when the tread wears level with them, the tire has reached the U.S. replacement limit

why do hiccups make that hic sound? your diaphragm suddenly pulls air in. then the opening between your vocal cords snaps shut

why are so many pencils hexagonal? six flat sides give your fingers purchase and make the pencil much less likely to roll off the desk

why is one nostril often clearer than the other? your nasal passages take turns. tissues on one side swell a little while the other opens for airflow

why is there a little plastic table in a pizza box? it props up the lid, so it does not press into the pizza and its toppings

why do your fingers wrinkle in water? water makes tiny blood vessels in your fingertips constrict. the skin folds into channels that may help with wet grip

why do qr codes have three big squares? they are finder patterns. they tell a scanner where the code is, which way it faces, and how large it appears

why does ice cream hurt your forehead? cold food can chill the roof of your mouth, while the pain is felt in your forehead that mismatch is referred pain

why does a tape measure hook move? it slides by its own thickness, so the zero point works from an outside edge or an inside wall

ice floats because freezing gives water a more open structure and a lower density. that floating layer also insulates liquid water and life below.

bar charts use length to represent value. cutting off zero can make a small difference look huge, so always check where the scale begins.

grapefruit can change the amount of some medicines in your body, sometimes raising it and sometimes lowering it. check the label or ask a doctor or pharmacist.

when a problem is new, study one complete worked example before solving another yourself. the example shows the reasoning, not just the answer.