Penrose Triangle, WikiPedia

Standing here
The old man said to me
“Long before these crowded streets
Here stood my dreaming tree”
Below it he would sit
For hours at a time
Now, progress takes away
What forever took to find
And now, he's falling hard
He feels the falling dark
How he longs to be
Beneath his dreaming tree
Dave Matthews Band, The Dreaming Tree

I'm home again in my old narrow bed
Where I grew tall and my feet hung over the end
The low beam room with the window looking out
On the soft summer garden
Where the boys grew in the trees
Carly Simon, Boys in the Trees

The Bermuda Triangle got tired of warm weather. It moved to Alaska. Now Santa Claus is missing.
Steven Wright

There's a pizza place near where I live that sells only slices. In the back, you can see a guy tossing a triangle in the air.
Steven Wright

Happiness Is Simple, Simon Franglen

The Dreaming Tree, Dave Matthews Band

Boys in the Trees, Carly Simon