To confirm if an egg is fertilized or not, this light will let you see then by showing a dark spot and veins. You just can't tell before then. You can mark your eggs with dates in pencil so that you can keep track. This light was very helpful and was really fun to watch the growth of the eggs. Features - Color: silver - Material: Stainless Steel and Nylon - Size: 10.00X1.00X1.00cm/3.93X0.39X0.39in -Note: powered by AAA batteries(not included) - Made from premium nylon and stainless steel material, it's durable for long time use. - By shining an egg candler torch through the shell, you can often make out signs of progress as and when they take place. - When checking whether the egg to be incubated is fresh or not, just put the bigger side of the egg to the upside, turn on the LED egg candler and aim at the tops of bigger side. - The light sources are available which do not carry the risk of harming the development of the embryos. - Light weight and practical egg candler is a product that checks whether the egg is fertilized and grows healthy.Packing list: 1 x egg candler