Evaluating Performance - How Do You Know If Your Neural Network Is Actually Smart?

So you've built a neural network, trained it with gradient descent, and watched the loss function steadily decrease. Success, right? Well, not so fast. A decreasing loss function is like a student who's getting better test scores - but what if they're just memorizing the specific questions instead of actually learning the material? This is where model evaluation comes in, and it's arguably the most important step in the entire machine learning process.