Ais a collection of modems and other equipment in a local area that acts as an isp's gateway to the internet.

An point of presence (PoP) is a collection of modems and other equipment in a local area that acts as an ISP's gateway to the internet.  ISP stands for Internet Service Provider - the company that connects the client through his communications line to its servers, or central computer, which connects client to the internet via another company’s network access points.   The PoP is the interface point between communicating entities. 

answer: when the substance contain the two parts (polar and non-polar) like soap.

explanation:

soap is produced from the basic hydrolysis of ester "saponification", it is considered as a salt of hydrocarbons. the soap molecule contains the two parts: a polar group   (-coo-na+) and a non-polar group (r-hydrocarbon part). the polar group is called the head and the tail is the non-polar group. the polar group is hydrophilic in nature (water loving) and the non-polar group is hydrophobic in nature (water repelling).the polar/non-polar nature of a substance is useful that it can dissolve the non-polar substances by the polar part and also can dissolve the polar substances by the non-polar part. it commonly used in detergents and soaps.