An example of a grad-school level question?
A balloon filled with helium gas is set free and starts to move upward. Which of the following best explains why the helium balloon moves upward?

An example of a grad-school level question?
A large tree was cut down with a saw. What is the best way to estimate the age of this tree?

An example of a grad-school level question?
In the atomic model that includes a nucleus, the positive charge is

An example of a grad-school level question?
A root emerges from a seed because of the force of