How can a snail get in a fish tank without ever putting it in there?

I some how magically obtain a snail in my fish tank I had never noticed it because it is the same size as the pebbles. I only recently had put in two sea shell into the tank, how is it possible that I have the snail?would the snail have came with the pebble?|||believe it or not....snail eggs are in tap water...yes thats right, we drink snail eggs and god knows what else. Even normal filters don't completely get rid of them...how gross eh....I had the same thing happen in my tank without adding fish, plants etc.|||yes they could of come with them but they can also come in plants as well i didnt know buying a few plants would me me having to pay out for snail remover i ended up emptying the tank of everything and substrate because of they rate they reproduce id say get it why thats it|||If you recently got plants or aquarium decorations from someone with a tank, there could have been snail eggs on them. Newborn snails are just little specks, smaller than a grain of salt so easy not to notice it's there until it gets a little bigger.