The first step in the scientific method is to have a question. Use any question you wish to gain the students' interest. A murder case is an easy option, and carries a sense of intrigue, allowing children to play detective in their minds. Tell them a story of a respected billionaire who was found dead in his house. Include facts and clues in your retelling. Include a couple of people with motives as suspects. Ask the children who they think is responsible for the murder. This section represents the initial scientific question.
The children will hopefully come up with some ideas in response to your question. This is a good time to introduce some research to clarify any unsure facts. Have the fictional detective interview the key suspects and find out where they were at the time of the murder. Include one fact which almost certainly makes one party out to be guilty. Scientists always research their topic before starting experimentation.
Ask the children who they think killed the billionaire, and why they think that. They should identify the guilty-looking party as the murderer. Write their hypothesis up on the board. Point out that it wouldn't be fair to convict the suspect on the basis of the currently known information, because he could have been telling the truth about his alibi --- or any other set of circumstances that could lead to the assumption being wrong.
To find out if the suspect really is guilty, a test should be conducted. A missing murder weapon is a good idea for this phase of the lesson. Explain that the detective searched the suspect's house and found the weapon, and that it matches the wounds found on the victim.
Ask the children if they can assume that the suspect is guilty based on the findings of the investigation. Explain to them that scientists use the same methods when conducting an investigation. Questions are addressed by research, followed by a hypothesis or several hypotheses being formed, and then they are tested. The results of those tests are used to determine whether the hypothesis is true.