Rational drug design is like building a Lego tower. First, you need to know what type of tower you want to build, and then you can start looking for the right Lego pieces to put it together. In the same way, scientists who want to make a new medicine first need to know what type of illness they want to treat, and then they need to find the right chemicals that can be put together in just the right way to make a medicine that works.
To make a new medicine, scientists first study the illness they want to treat. They try to find out what makes a person sick and what kinds of things can help them feel better. Then they start looking for chemicals that might be able to help. They might start by testing many different chemicals to see if any of them can help.
Once they have found some chemicals that might work, then they use a computer program to study how the chemicals work and to see if they can change the chemicals in certain ways to make them work better. It's like playing a video game where you can try different things to see what works best. Scientists can use these computer programs to help them decide what changes to make to the chemicals to make them work better.
After they make a new chemical that they think might work as a medicine, then they test it again to see how well it works. They might test it on cells in a laboratory, or on animals to see if it makes the animals feel better. If it works well and doesn't cause any bad side effects, then they can start testing it on people who are sick with the illness they want to treat.
Finally, if the medicine works well and doesn't cause any bad side effects in people, then it can be made into a medicine for everyone to use to help them feel better. It's like building a Lego tower that you can share with your friends when you're done!