What is the key characteristic of a funnel used with centrifuge tubes?

The key characteristic of a funnel used with centrifuge tubes is that it facilitates the transfer of liquids. Funnels are designed to allow for the smooth pouring of liquids into containers with small openings, such as centrifuge tubes, without spilling. This function is essential in laboratory settings, where precision and cleanliness are paramount in experiments and procedures involving liquid samples. By directing the flow of the liquid efficiently, funnels help maintain accuracy in measurements and reduce contamination.

Other options, such as weighing small animals or measuring liquid weight, do not pertain to the primary function of a funnel in this context. Similarly, while incising skin relates to surgical procedures, it is irrelevant to the function of a funnel or a centrifuge tube. Hence, the role of the funnel in aiding the transfer of liquids is what makes it a crucial tool in laboratory operations involving centrifugation.