Ms. Frizzle is placing the seaish rock with BraveBean's beanie on the top of the cairn.
… the title text?
Spoiler:
Earthling on Mars and moody7277 did. (Ms. Frizzle is aware of the invisible danger. She's handling the rock with BraveBean's contaminated beanie carefully enough that she doesn't touch or inhale any plutonium oxide.)