A genetically modified organism (GMO) is when scientists take the pieces of the genes in one kind of organism, like a plant or an animal, and put them into another kind of organism.
Imagine you have a Lego set with different pieces. You can create different things with them like a spaceship or a car. Scientists can do the same thing with genes by taking pieces of one organism and putting them into another to create something new.
One reason scientists do this is to make the organism stronger or healthier. For example, they might take a gene from a fish that can survive in cold temperatures and put it into a tomato plant so that it can grow better in colder weather.
However, some people are worried that GMOs might not be safe to eat or might harm the environment. So, scientists have to do a lot of testing to make sure that GMOs are safe before they can be sold in stores.
In summary, GMOs are organisms whose genes have been changed by scientists to create something new, with the main goal of improving their survival or health.