Unless you know how, taking out the core of an avocado can be messy. Do it like this:
Cut the avocado in half, around the core, and take away the half that is loose.
Hold the half with the core in one hand and tap the core two or three times with the back of the knife handle to loosen the core.
Take the core out by hitting it across the middle with the knife blade and twisting the knife left or right. Be careful not to cut yourself!