Questions

The capacity of a water heater is measured in Gallons or Liters. In the USA, it is measured in Gallons and in India, we measure it in Liters.

Hence, if the capacity of the water heater is 15 gallons or 15 liters, then it says that it can hold 15 gallons or 15 liters of water.

Oligopoly is market structure where a small number of large firms are there. Oligopolistic industries are characterized by a few dominant firms and substantial entry barriers.

IPv6 - Internet Protocol Version 6.

IPv6 addresses have a size of 128 bits.

Internet Protocol Version 6 [IPv6] is the successor to the Internet's first addressing infrastructure, Internet Protocol version 4 (IPv4).

In contrast to IPv4, which defined an IP address as a 32-bit value, IPv6 address have a size of 128 bits. 

IRQ-0 is the system timer commonly use in all computers. In a computer, IRQ is an interrupt request which acts as a hardware signal sent to the processor that temporarily stops a running program and allows a special program, an interrupt handler, to run instead.

Therefore the answer for What travels around the world but stays in one spot is STAMP or any other thing that acts as a stamp does.

Ex. the letter in the envelope etc...

Since, Several things have the possibility, but the word travel evokes that it would move.