Tensile: If a long metal wire were hung from the ceiling with a weight on one end, the weight would place a force on the wire. Inside the wire, the force would be spread evenly through the whole wire. If enough force or weight were put on the end of the wire it could cause the wire to stretch. The atoms of the wire are being pulled apart by the weight or force on the wire.

Compressive: If the wire were a thin metal rod on a firm (hard) surface with a weight on the top, it would spread the weight (or stress) across the whole rod. Here the atoms of the rod are being pushed together.

Shear: The shear stress can be illustrated by using two blocks of wood nailed together. If one block of wood was pushed to the right and the other block of wood was pushed to the left, the nails would experience a shear stress at the point where the two blocks of wood meet. If pushed hard enough, the nails could bend or break.

If an airplane component has any one of the three stresses above, it could deform (bend) or even break. Engineers try to prevent this by choosing airplane parts that are strong enough. The parts must be strong enough to take extra stress, such as gusts of wind, downdrafts or turbulence (bumpy air).