Button won't go low.

I'm using an Arduino Uno, which I've programmed as a bike computer. I wanted to add a button to pause the LCD so data can be collected from the screen.

I put two pins of the button on a 5V rail (on a protoshield) and connected one of the others to digital pin 10. I think programmed the button just to control the LED on the board. When I turn the board on, the light is always on without me even touching the button. I tested continuity and pin 10 shouldn't be getting any power at all but it is.

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In addition to the very professional looking example that @larryd posted, I think something like this would be good enough. Oh, telling us the value of the pull down resistor would be good to.