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

[Q]: Passage: When water freezes in a non-crystalline, amorphous form as frost, the water molecules are fixed in place, but not in a regular, repeating pattern. Frost and other amorphous solids are characterized by their unorganized and unpredictable structure. Because ice comes in both amorphous and crystalline forms, there are differences in the physical properties of ice such as its hardness and density. Answer: amorphous
[A]: What kind of solid is characterized by an unorganized and unpredictable structure?

[Q]: Passage: tobacco smoke causes fatal lung disease in nonsmokers. Answer: tobacco smoke
[A]: What still causes fatal lung disease in nonsmokers?

[Q]: Passage: Mammals have separate sexes and reproduce sexually. They produce eggs or sperm and must mate in order for fertilization to occur. A few mammals are oviparous. They lay eggs, which later hatch. These mammals are called monotremes. Most mammals are viviparous and give birth to live young. These mammals are either placental mammals or marsupials. Placental mammals give birth to relatively large and well-developed fetuses. Marsupials give birth to smaller, less-developed embryos. In both placental and marsupial mammals, the young grow and develop inside the mother's body in an organ called the uterus. At birth, they pass through a tube-like organ called the birth canal, or vagina. Answer: monotremes
[A]:
Mammals that lay eggs are called what?