Valid means that you are testing what you think you are testing. Reliable means that you have not made any mistakes in your method and you have tested with repetition. Another person should be able to follow your method and get the same results.
Having more than one copy of the investigation is important in the reliability of an experiment. This can be done by replication or repetition. Some experiments can be repeated with the same procedure and material (for example, measuring the height of a bouncing ball). This is non-destructive testing. Other experiments, such as germinating seeds, cannot be repeated because the seeds have already germinated. This is destructive testing. In destructive testing replication is used and many seeds are germinated.
In this investigation you could use repetition of the experiment to make sure your results are reliable.