This is a jar of pickled cucumbers. A pickled cucumber is a usually small cucumber that has been pickled in a brine, vinegar, or other solution and left to ferment for some time. This is done by either immersing the cucumbers in an acidic solution or through souring by lacto-fermentation. Pickled cucumbers are often part of mixed pickles. It is often claimed that pickled cucumbers were first developed for workers building the Great Wall of China, though another hypothesis is that they were first made in the Tigris Valley of Mesopotamia, using cucumbers brought originally from India. Like pickled vegetables such as sauerkraut, sour pickled cucumbers (technically a fruit) are low in calories. They also contain a moderate amount of vitamin K, specifically in the form of K1. A 30-gram sour pickled cucumber offers 12–16 µg, or approximately 15–20% of the Recommended Daily Allowance, of vitamin K. Most sour pickled cucumbers are also high in sodium. Sweet pickled cucumbers, including bread-and-butter pickles, are higher in calories due to their sugar content. Pickles are being researched for their ability to act as vegetables with high probiotic content. Probiotics are typically associated with dairy products, but lactobacilli species have been shown to add to the nutritional value of pickles.