Things that were invented for some other purpose  , But it started being used for some other work. 

Number 1:- bubble wraps

Now these bubble  wraps which are used in packaging , Do you   know that , originally it was used to apply 3D  wallpaper on the wall , But that idea failed,   and later it started being used in packaging.

Number 2 Trademill,

Now a days people use   treadmill to burn calories , But do you  know that, it was originally invented   to punish prisoners, Actually William  Cubbitt, the inventor of the treadmill,  once visited a prison , And there he saw that,  all the prisoners were sitting motionless , And   an idea came in his mind, why not make a treadmill  , prisoners will run on it ,and in the same way   workout will also be done, and from that we  will be able to generate electricity as well. 

Number 3 Coca-Cola,

Coca-Cola was invented  specifically for those soldiers , who took   morphine to heal their wounds , But  the soldier did not buy Coca-Cola , and   later it was converted into a cold drink.

Number 4 chainsaw,

Chainsaw is used today to cut   trees, But you will not believe, It was originally  invented to perform surgery during pregnancy.