Science grows like a weed every year.

Taken at face value, the line suggests that scientific knowledge expands rapidly and often uncontrollably—“like a weed” implies vigorous, opportunistic growth that can outpace careful cultivation. Attributing this to Mullis (who was famously skeptical of scientific consensus and critical of institutional science) can also color the meaning: it may hint at his view that science produces an ever-thickening mass of results, papers, and claims, not all of which are equally valuable or well-tended. The metaphor can be read ambivalently—celebrating science’s fertility while warning that growth alone is not the same as wisdom, rigor, or discernment.