The onClick property is an event handler that notifies you when the mouse has been clicked on the button.
<html>
<head>
<title> Example of the radio onClick event handler</title>
</head>
<body>
<form name="form1">
<input type="radio" name=button1
onClick='alert("This mouse was clicked on this object")'>Box 1
<br>
</form>
</body>
</html>