The most common, and most purely linguistic, criterion is that of mutual intelligibility: two varieties are said to be dialects of the same language if being a speaker of one variety confers sufficient knowledge to understand and be understood by a speaker of the other; otherwise, they are said to be different languages. However, this definition becomes problematic in the case of dialect continua, in which it may be the case that dialect B is mutually intelligible with both dialect A and dialect C but dialects A and C are not mutually intelligible with each other. In this case the criterion of mutual intelligibility makes it impossible to decide whether A and C are dialects of the same language or not. Cases may also arise in which a speaker of dialect X can understand a speaker of dialect Y, but not vice versa; the mutual intelligibility criterion flounders here as well.
If it is possible to answer this question, answer it for me (else, reply "unanswerable"): Which is the most uncommon and purely linguistic criterion?
Ah, so.. unanswerable

Although research has been inconclusive, some findings have indicated that electronic communication negatively affects adolescents' social development, replaces face-to-face communication, impairs their social skills, and can sometimes lead to unsafe interaction with strangers. A 2015 review reported that "adolescents lack awareness of strategies to cope with cyberbullying, which has been consistently associated with an increased likelihood of depression." Studies have shown differences in the ways the internet negatively impacts the adolescents' social functioning. Online socializing tends to make girls particularly vulnerable, while socializing in Internet cafes seems only to affect boys academic achievement. However, other research suggests that Internet communication brings friends closer and is beneficial for socially anxious teens, who find it easier to interact socially online. The more conclusive finding has been that Internet use has a negative effect on the physical health of adolescents, as time spent using the Internet replaces time doing physical activities. However, the Internet can be significantly useful in educating teens because of the access they have to information on many various topics.
If it is possible to answer this question, answer it for me (else, reply "unanswerable"): Why do research findings believe the Internet has a negative effect on teen physical health?
Ah, so.. time spent using the Internet replaces time doing physical activities

Very low sounds are also produced in various species of Coleoptera, Hymenoptera, Lepidoptera, Mantodea and Neuroptera. These low sounds are simply the sounds made by the insect's movement. Through microscopic stridulatory structures located on the insect's muscles and joints, the normal sounds of the insect moving are amplified and can be used to warn or communicate with other insects. Most sound-making insects also have tympanal organs that can perceive airborne sounds. Some species in Hemiptera, such as the corixids (water boatmen), are known to communicate via underwater sounds. Most insects are also able to sense vibrations transmitted through surfaces.
If it is possible to answer this question, answer it for me (else, reply "unanswerable"): Insect moving sounds are used to warn and do what with other insects?
Ah, so..
communicate