"To break an egg after ye meate is out we are taught in our childhood... and the intent thereof was to prevent witchcraft; lest witches should draw or prick their names therein and veneficiously mischiefe ye persons, they broke ye shell... This custome of breaking the bottom of the eggeshell is yet commonly used in the country"

Sir Thomas Browne, Vulgar Errors, 1686

Breaking the bottom of an eggshell after you have finished with the egg is an old English custom, that to some extent is still practiced today. When I was younger my father told me that witches would become trapped inside eggs. If you broke open the bottom they would escape, and bring you good luck as a manner of thanks. I still do this even today. Oddly, once the hole has been made you should immediately discard the eggshell, as I have it on good authority that prolonged exposure could give you warts.

Another story tells us that witches sail in eggshells. Putting holes in the bottom would sink them, and stop them causing shipwrecks. There is an anonymous poem dating from the 1930s that accompanies this:

Oh, never leave your eggshells unbroken in the cup,
Think of us poor sailor-men and always smash them up,
For witches come and find them and sail away to sea,
And make a lot of misery for mariners like me.

Reference: Oxford Dictionary of English Folklore, Jacqueline Simpson and Steve Roud.