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One of the most amazing and confounding tools in Adobe Illustrator is probably the Gradient Mesh tool 
. Working with this feature allows the user to divide almost any shape into a mesh. Each intersection of two lines (horizontal/vertical) can be colored with a solid color - Useful guidlines:
- Determine when it is best to use:
Use a linear Gradient :
Use the Mesh for smoke effect:
- Don't keep on adding points when things get out of hand...be very methodical
- Use either the Mesh tool or the Direct Selection tool to reposition points
- Tools:
- Gradient Mesh tool
- Direct Selection Tool
- Eyedropper tool
- Additional Reading
- Gradient Mesh for Beginners
- Portraits (This one is extremely difficult, but good to get an idea on how to become an expert)
- Mesh tutorials