javascript setInterval(tick(), 1000)

Question

The following is my code, it will call tick(a function which will print something on console) per second

function tick() {
    console.log("djfiah");
}
windowa.onload(){
    var id = setInterval(tick, 1000);
}

If the code become

var id = setInterval(tick(), 1000);

Why the tick will be called only once?