function enterNumber(){
    var number = parseInt(prompt("Enter your number"));
    if (isNaN(number)){
        alert("Please enter a valid number");
    }
    else {    // test what is entered is a number
        numbers.push(number);   //add the number entered to the end of the array
    }
    document.getElementById("numberList").innerHTML = "The numbers you have entered so far are: " + numbers;  //diplay a list of number entered
    enterButton.textContent = "Enter your next number"; //change the label on the Enter Button
    calculateButton.style.visibility = "visible";     //show the button  - uses the CSS property of the elenment
}