What thing in our body decides the entry of foreign particle

The nose, mouth, ears and eyes decide the entry of foreign particles. Little children mostly acquire it because mostly children put their hands or stick their tongues on nearby objects which are mostly infested with bacterias and pathogens, sometimes these particles are harmful and sometimes they aren't.

The speed of  sound:   sound travels  at different speeds depending on what it istraveling through. of the three mediums (gas, liquid, and solid)  sound waves travel the slowest through  gases, faster  through  liquids, and fastest  through  solids.