0610 GMT March 22, 2018
While most are small and pass out painlessly when you urinate, some can be more serious, express.co.uk wrote.
According to the National Health Service (NHS), they can cause a kidney infection with unpleasant symptoms such as a high temperature, chills and diarrhea — and even sepsis.
They're usually caused by a build-up of particular chemicals in the body.
One of these is oxalic acid which can be found in spinach — a popular food for many a healthy dieter.
Sandra Greenbank, a nutritional therapist, said, "Raw spinach contains oxalic acid, which has the ability to bind to minerals in our intestines to form oxalates — or insoluble salts.
"Oxalates are found in kidney stones and some people who are genetically predisposed to kidney stone formation may need to be careful with consuming too many foods high in oxalic acid."
What's more, oxalic acid is also said by experts to block absorption of spinach's rich amounts of iron, potassium, vitamin A and calcium.
It's attributed as the reason for why studies have shown that as little as two percent of the iron from spinach is actually absorbed by the body.
However, spinach isn't the only food to contain oxalic acid.
Greenbank explained, "As well as green vegetables, swiss chard, beetroot tops and parsley have high amounts too.
"As can almonds, cashews, buckwheat, tea, coffee and chocolate.”
If you're keen to keep spinach in your salad, Greenbank recommends choosing baby spinach.
She explained, "Young vegetables are less likely to have as much as it tends to increase as foods mature."
Fortunately, oxalic acid can be destroyed by using heat, such as steaming.
However, Greenbank highlights another reason why you may want to always cook your spinach.
She said, "The other potential problem with raw spinach is the possible contamination of Listeria, Salmonella or E. Coli bacteria, which can cause serious disease.
"They can be killed by cooking — steam spinach for about four minutes before eating it.
"While it poses a low risk of infection, pregnant women, the elderly, small children and those with a suppressed immune system could be at risk of potentially serious illness."