Lines, shapes, piece shape, pattern, and color are the foundational elements that determine the difficulty of a jigsaw puzzle.

The number/quantity of details in an artwork that comprise these basic elements can be increased or decreased to make a puzzle easier or harder. More lines doesn't necessarily mean a puzzle will be harder to solve, same with color. But the combination of any of these elements typically means you will have an easier time solving a given puzzle. However, when you begin removing one or more of these basic elements, it typically increases the difficulty. 

The average puzzle incorporates all of the basic elements to one degree or another, which makes the overall experience easy. Of course, piece count plays a roll too, but it's not relevant to this discussion. Why? Increasing the number of pieces doesn't necessarily mean a puzzle will be more difficult to put together. It will take you longer to put a 5,000 piece puzzle together than a 1,000 piece puzzle, but it doesn't automatically make the image more difficult. 

Gradient puzzles are very popular, but they aren't easy. They're pretty. A standard gradient puzzle forces you to rely on two elements to put them together: Piece shape and color. Some gradient puzzle designs include line and shape to help make the puzzle easier. 

Flat-color puzzles have exactly one means of solving the puzzle: Piece shape. These are, by all reasonable rationalizations, the most difficult puzzles to solve.

Why are so many puzzlers afraid to tackle a black and white puzzle? They're just puzzles, after all.

In a word: Color. There is none. The single most important element most puzzlers rely on to solve a puzzle is gone. The dingo done ate your baby. 

The challenge, however, isn't the fact that color is missing ... It's that your brain can't grok how to devise a plan of attack. How do you separate pieces, create groupings, or enjoy a puzzle when your crutch is snatched away from you? 

Despite the lack of color, my animals puzzles provide a wide assortment of clues how to solve the puzzle despite the lack of color and standard piece shapes. Lines, shapes, and patterns are the keys. There are literally thousands of details within the artwork that will guide the way—line width, line shapes, patterns of all kinds. Look for odd shapes within the art, eyes, feet, beaks, horns, stripes, dots, fur, squiggles, straight lines, and more to help you solve the puzzles. Your brain will figure it out ... and enjoy the process.

OddGoods animal puzzles are definitely challenging. And for some, maybe the most difficult puzzles they will ever take on. However, you haven't felt a sense of accomplishment quite like the feeling you will get after placing the last piece of one of my black and white puzzles: choose from birds, cats, and big fish. That being said, I do recommend having a bottle of wine nearby.