stare, stares, stair, stairs

Stare or stare means to look at someone or something intensely, with a fixed gaze--
His mother told him that it was rude to stare.
Don't stare! It makes me nervous.
Stares is the third person singular ( he, she, it ) form of stare--
He stares when he is thinking about something.
I don't like the way she stares at me.
Stare may also be a noun ( thing or idea )--
She mocked him with an insolent stare.
His stare got everyone's attention.

Stair or stair ( rhymes with stare ) is an old-fashioned word for stairs, meaning the steps from one floor of a building to another, or any one step of the set--
"Yesterday upon the stair, I met a man who wasn't there..."
We should get some stair treads for the steps.
Stairs means a set of steps that go from one floor or level to another--
Be careful on the stairs.
He fell down the stairs and broke his leg.

Now that you know that, you can say--
"He always stares when she comes down the stairs."
"What is making you stare? Is there something on the stair?"

Note--"upstairs" means on the floor above--
Don't play ball upstairs. It's too noisy.
Where is he? He's upstairs.
"Up the stairs" means walking up the steps to the second floor, or the floor above--
Don't run up the stairs. You'll fall.
"Downstairs" means on the floor below--
I wish the people downstairs would be quiet.
Mom went downstairs a minute ago.
"Down the stairs" means walking down the steps to a lower floor or level--
Walk down the stairs slowly, or you might fall.

Note # 2-- to "stare down" means to stare at someone until he sees your point of view--
They made fun of him, but he was determined to stare them down.

Note # 3--a "staircase" is a set of steps --
Don't leave your toys on the staircase.