Rather than purchasing a weed killing product from the shops, you can use rock salt (or any kind of salt) to accomplish the same task. Rock salt will prevent the weeds from absorbing water from the ground and cause them to die. However, you should exercise caution when using rock salt to kill weeds that live among other plants. Using too much salt can kill the other plants in addition to the weeds you wish to get rid of.
Drop a small pinch of rock salt at the base of the weeds, near the roots, that you wish to kill. After a bit of rainfall, the salt will dilute and allow other plants to grow back in this area -- making this amount of salt safe to use around other plants.
Mix a solution of rock salt and water to spray onto the weeds you wish to kill as an alternative method. To create this solution, boil water in a kettle, dissolve 200 g (1 cup) of rock salt into 474 ml (2 cups) of boiling water. After cooling, pour the solution into a spray bottle and apply to the weeds you wish to kill.
Pour a larger amount of rock salt (or saltwater solution) onto areas that you do not wish for plants to ever grow in, such as cracks in paths or gravel driveways.