You don't have to tell me I already know
That money on bushes or on trees does not grow
Or dogs can't play guitars or worms can't sing
Or an eagle can't soar after breaking a wing.

You don't have to tell me that the dodo is dead
Or that mad-man King George was not right in the head
And that all forms of life are born to die
This is common knowledge and facts do not lie.

You don't have to tell me that elephants can't fly
Or that rhinos can't dance on the clouds of the sky
Or that a full grown shark looks much like a trout
What I speak of here surely goes without doubt.

I may not be the brightest but I too can tell
The difference between the sound of a car horn and a church bell
And I am not as silly as I may appear
For I too don't believe everything I do hear.

Francis Duggan

http://www.poemhunter.com/poem/you-don-t-have-to-tell-me/