Given a scientific passage and an answer, generate a question for the given answer.

[EX Q]: Passage: Hair, hair follicle, and oil glands. The oil, called sebum, helps to prevent water loss from the skin. The sebaceous gland secretes sebum, which waterproofs the skin and hair. Answer: sebum
[EX A]: What does the sebaceous gland secrete which waterproofs the skin and hair?

[EX Q]: Passage: Solutes change the physical properties of solvents. They lower the freezing point and raise the boiling point of solvents. Answer: solutes
[EX A]: What change the physical properties of solvents?

[EX Q]: Passage: 21.4 DC Voltmeters and Ammeters Voltmeters measure voltage, and ammeters measure current. A voltmeter is placed in parallel with the voltage source to receive full voltage and must have a large resistance to limit its effect on the circuit. An ammeter is placed in series to get the full current flowing through a branch and must have a small resistance to limit its effect on the circuit. Both can be based on the combination of a resistor and a galvanometer, a device that gives an analog reading of current. Standard voltmeters and ammeters alter the circuit being measured and are thus limited in accuracy. Answer: voltage
[EX A]:
What do voltmeters measure?