A long-term experiment is like when you plant a seed and watch it grow over a really long time. Instead of just checking on it a few times and then forgetting about it, you keep watching it for weeks, months, or even years to see how it changes over time.
It's like when your parents measure how tall you are every year on your birthday. They do this to see how much you have grown. A long-term experiment is similar. Scientists use long-term experiments to study how things change over a long period of time. They want to know what happens after a day, a week, a month, or even years pass.
Long-term experiments can be used to study lots of different things like plants, animals, or even people. For example, if scientists want to know how a new medicine works over a long period of time, they might do a long-term experiment. They would give some people the medicine and others a fake one (called a placebo), and then watch what happens for a long time.
Overall, long-term experiments are like watching something grow and change over a really long time so that scientists can understand how things work over time.