Add a lovely reflection to egg yolk is really fun. The reflection is vivid which can bring an unusual experience for you. After learn this tutorial, you can make more reflection on other stuff, like the surface of coffee and milk etc.
This is the egg I’ll use in the tutorial. 
And the following is the effect. 
A little bit cute? Ok, let’s begin.
Step 1: Open this picture and choose lasso tool to make a selection of Egg Yolk.
Step 2: Click New Layer to make a transparent new layer. Then, click Brush Tool and choose Black color in the Swatch to fill the selection with black color.
Step 3: Make the lovely stuff we wanna make it on the Egg Yolk.
Open the stuff as a new layer.
And then, Move it down to under Layer 1. Next, choose Free Transform Tool to make it a proper size. The part under the black will be shown on the Egg Yolk.
Step 4: Make Layer 1 invisible, and choose “Select > Feather” to feather the border of this selection.
Set the parameter to 5.
Step 5: Delete the other part of the selection.
Choose “Select > Inverse”, and press Delete on the keyboard. Now, there leaves only one part.
Step 6: Press “Ctrl + D” and adjust the Blending to Soft Light. 
Now, we’re successfully accomplished!