In just a few short lines, the speaker describes the moment a young person starts to understand what science can be. A good experiment, they realize, isn't a dry procedure but a "mystery play," something closer to theater or ritual, where the outcome is hidden and the act of watching matters. It is also "a game," which suggests curiosity and fun rather than duty.
The most interesting moment is the idea of having a head "free to be troubled." Being troubled sounds unpleasant, but here it seems like a gift: the speaker welcomes confusion and open questions as part of the pleasure. The short last line, "And I am one," lands quietly and with confidence. The speaker claims the title from the poem's name, a budding scientist, and seems proud of it. The mood is light, curious, and quietly joyful, with a sense of someone discovering that not knowing things can be exciting. The brief lines and breaks mimic the small, careful steps of learning.