Lye, also known as sodium hydroxide, is used in all soaps. The result of a chemical reaction between sodium carbonate and calcium hydroxide, lye causes soap to saponify, or harden. If you don't add lye to a soap mixture, you'll end up with detergent rather than hardened soap. If you make homemade soap, you have to be extremely careful when handling lye, as it can be very harmful. Thankfully, you can avoid the risks of handling lye by repurposing store-bought soap to make your own homemade olive oil soap.