NSES Content Standard A 
Science as Inquiry: Abilities necessary to do scientific inquiry 
Grades K-4, page 122 

Plan and conduct a simple investigation. In the earliest years, investigations are largely based on systematic observations. As students develop, they may design and conduct simple experiments to answer questions. The idea of a fair test is possible for many students to consider by fourth grade. 

Benchmark 1B The Nature of Science: Scientific Inquiry
Grades K-2, page 10
Describing things as accurately as possible is important in science because it enables people to compare their observations with those of others.

Benchmark 1B The Nature of Science: Scientific Inquiry
Grades 3-5, page 11
Scientific investigations may take many different forms, including observing what things are like or what is happening somewhere, collecting specimens for analysis, and doing experiments. Investigations can focus on physical, biological, and social questions.

Benchmark 12A Habits of Mind: Values and Attitudes
Grades K-2, page 285
Raise questions about the world around them and be willing to seek answers to some of them by making careful observations and trying things out.

Benchmark 12E Habits of Mind: Critical-Response Skills
Grades 3-5, page 299
Recognize when comparisons might not be fair because some conditions are not kept the same.