Final answer:
A scientist's initial explanation for observed facts is known as a hypothesis, which is a testable and preliminary explanation to guide further investigation.
Step-by-step explanation:
A scientist's first try at explaining observed facts is usually called a hypothesis. A hypothesis is a tentative explanation for scientific observations that puts the system being studied into a form that can be tested. It acts as a guide for gathering and checking information during experimentation. An example of forming a hypothesis might be a scientist noticing that certain plants grow taller in one type of soil compared to another, leading to the hypothesis that soil type affects plant growth.