Final answer:
Rats can enter buildings through very small openings, about 20 millimeters in diameter, which is similar in size to a microscopic house mite.
Step-by-step explanation:
Rats are known for their ability to squeeze through incredibly small spaces to enter buildings. Rats can infiltrate through gaps and holes that are as small as a quarter of their body length due to their flexible skeletons. For example, most adult rats can fit through a hole that is about 20 millimeters (or 0.75 inches) in diameter, which is roughly the size of a US quarter coin.
It's interesting to note that the size of these openings is similar to the size of a microscopic house mite, which is approximately a half a millimeter in size. This comparison helps to illustrate just how small an opening a rat can utilize to gain access to a sheltered area, making it a challenge to rat-proof a building.