Pipe Dream is a little odd. You have to lay down some pipes to create a path for some kind of liquid to run through. The thing is, though, that the liquid flows as you're laying down the pipes, and if it dribbles out the end your progress is stopped, and your score is tallied. So, it's a high-pressure situation.

Unless...

Unless you press (and hold) Start to pause the game

And while you're holding Start, press

A, B, B, A

Then, unpause.

Done correctly, pressing the B button will stop the ebb of fluid, and allow you to build pipes to your heart's content.

Then you just press Select once you've created your masterpiece to let the fluid flow again.