Defining 'onclick' when appending a button in JS

Question

var button1 = document.createElement("button");
button1.innerHTML = "TEST BUTTON";
button1.onclick = alert("Knock-knock");
document.body.appendChild(button1);
Why do I get alerted just when visiting/updating the page, but not when clicking on the button?
And what should I do to achieve desired behavior (getting alerts only when clicking on the button)?
Thank you in advance for answering.