Step 1: What a Shamrock Looks Like
It has three leaves. Thats 3. not 4 or 2 or 792. Its 3.
Each of them are a very pretty heart shape. Thats a lotta love.
St Patrick used it to explain to the pagan Irish that God is a Holy Trinity. Three in one. Like this wee plant. Three leaves in one plant.
We Irish caught on quickly to this explanation and thus stopped worshiping the moon, conjuring spirits and we all became Christian. Thanks Paddy.
The first guy that came along and tried to make us christian was swiftly killed a short while after arriving in Ireland. But Partick knew the old shamrock trick. We Irish like shamrocks.
To see what the first guy used and what ended up getting him killed :-P , look at the next step in this instructable.
Step 2: The Cursed Four Leaf Clover
St Patrick didn't use it because its a four leaf clover and NOT a Shamrock.
Using this as a St Patricks Day symbol is just stupid.
Turn up in Ireland on St Patricks Day, or any day, with this thing on your t-shirt and you'd better know where the nearest open door or window is. :-P Joke.
That or have a good excuse, like,
your the chairperson for the four leaf clover appreciation society and you've been out all day looking in fields for one; and all you want to do now is sit down, have a beer and look at a Shamrock because your sick to death of looking at stupid 4 leafed things and shamrocks are so much better because they have 3 cute heart shaped leaves and is the symbol of St Patrick and isnt a stupid 4 leaf clover.
Learn that off by heart and you may survive.
Actually we Irish are more likely to have a laugh with you at your expense, poke some fun and buy you a drink to make you feel better for being so stupid. And because we feel sorry for someone who's the chair of a four leaf clover appreciation society.
Step 3: Re-cap
A Shamrock has 3 heart shaped leaves and is all you will be painting on your faces from now on.
You've been fantastic, God bless ya and have a fantastically happy St Patrick's Day.