Photo/foodie.com
Onions absorb bacteria and that is the reason why they are so good at preventing us from getting colds & flu. And for that very reason, we should NOT eat an onion that has been sitting for a time after it has been cut open.
Left over onions are poisonous. When food poisoning is reported, the first thing the officials look for is whether the 'victim' ate onions and where those onions came from. Most probably the onions are the cause.
Onions are huge magnets for bacteria, especially uncooked ones. Never ever keep a portion of sliced onion over a period of time and thereafter use it to prepare food. It's not even safe if you put it in a zip-lock bag and placed it in your refrigerator. It is still poisonous
Also, do not give onions to dogs. Their stomachs cannot metabolize onions.
Please remember it is dangerous to cut an onion and try to save it to cook with the next day, it becomes highly poisonous for even a single night - as it creates toxic bacteria which cause adverse stomach infections because of excess bile secretions and even food poisoning.