Given scientific fact as input, generate the question from this fact such that it can be answered from the input.

[EX Q]: The nuclear envelope is a double-layered plasma membrane like the cell membrane, although without membrane proteins.
[EX A]: The nuclear envelope is a double-layered plasma membrane like the cell membrane, although without what?

[EX Q]: There are three quarks in each proton and neutron.
[EX A]: How many quarks are in each proton and neutron?

[EX Q]: The lower density of warm air causes it to rise above cold air.
[EX A]:
What property of warm air causes it to rise above cold air?