Detective Work explainer

The aim of this puzzle: Make the sign turn black when you tap on it.
Walkthrough of the solution: shape.on('click', messageRevealer); will run the messageRevealer function when you tap on the shape (rectangle), but the function is empty. First, start with the object you want to modify: shape. Next, you are going to try to change its color which is an attribute. You change attributes using .attr() which takes two arguments: the attribute name and the value you want to give that attribute. The color attribute is called the 'fill', and you want the color to be 'black'. All of that gives you shape.attr('fill', 'black'); Just remember that it should be in between the curly brackets of your messageRevealer function.
Sample code solution:
(Tap below to reveal)

shape.on('click', messageRevealer);
function messageRevealer(){
  shape.attr('fill', 'black');

JavaScript Concepts: Code Block (function), Identifiers, Member Expression
D3 Concepts: .on(‘click’,), .attr(‘fill’,)