In this task, you are given a question and an answer, you would be asked to create the sentence based on the Question-Answer provided. It should be contained within the Question-Answer provided.

Question: What occurs before the endometrium thickens in estrous cycles? Answer: ovulation
Ovulation occurs before the endometrium thickens in estrous cycles.

Question: What is the third male reprodutive structure after the penis and testes? Answer: epididymis
Epididymis is the third male reprodutive structure after the penis and testes.

Question: What term describes the number of items in a given area or volume? Answer: concentration

The term concentration describes the number of items in a given area or volume.