Toughness: When a huge external force is applied on a metal, the metal will experience fracture. Toughness is the ability of a metal to resist fracture. The ability of a material to resist fracture due to high impact loads, is called toughness.

The toughness of a material decreases when it is heated.

The number of slots in a 25 mm castle nut is 6.

Efficiency of a self locking screw is 50%, for efficiency greater than 50% the screw is said to be overhauling screw.

The application of third type levers is found in a pair of tongs.
According to where the load and effort are located with respect to the fulcrum, there are three types or classes of lever:

* First-Class Lever
* Second Class Lever
* Third Class Lever

First Class Lever This is a type of lever which has the fulcrum in between the weight and the force applied. Its order is represented as force-fulcrum-weight. This is the most basic type of lever.

Example:
*Our hand pushing an object or seesaws, crowbars.
* Using scissors represents the use of two first-class levers.
* A wheel and axle is also an example.
* Pulling a nail out of a wooden plank also represents a first-class lever.

Second Class Lever In this, the fulcrum is at one end and the force applied is on the other end. The weight is situated in the middle of these two. The order of this would be fulcrum-weight-force. The application of force at one end will result in some work done on the other end.

Example:
Wheelbarrow
* Staplers
* Doors or gates
* Bottle openers
* Nutcracker
* Nail clippers

Third Class Lever These are the levers in which the fulcrum is at one end and the force is applied in the middle and the weight is on the other end. The order is represented as a weight-force-fulcrum. In this case, we have to apply more energy to displace the weight to a longer distance.

Example:
* Fishing rod
* A broom
* A baseball bat
* A bow and arrow
* Human jaw