The universe is the whole of space and all the stars, planets, and other forms of matter and energy in it. The Universe can be defined as everything that exists, everything that has existed, and everything that will exist. According to our current understanding, the Universe consists of spacetime, forms of energy (including electromagnetic radiation and matter), and the physical laws that relate them.

A black hole is a place in space where gravity pulls so much that even light can not get out. The gravity is so strong because matter has been squeezed into a tiny space. This can happen when a star is dying. Because no light can get out, people can't see black holes. They are invisible.

1. Inaudible sound: Human ear cannot detect sound frequencies less than 20 vibrations per second i.e. 20 Hz. So any sound below this frequency will be inaudible sound for humans. In the high-frequency range, the human ear cannot detect frequencies above 20000 vibrations per second (20 KHz).  So the frequencies below 20 Hz and above 20 KHz comes under the category of inaudible frequencies. The low-frequency sound which human ear cannot detect are also known as infrasonic sound. Whereas the higher range inaudible frequency are also known as ultrasonic sound.
2. Audible sound: Human ear can easily detect frequencies between 20 Hz and 20 KHz. Hence sound waves with frequency ranging from 20 Hz to 20 KHz is known are audible sound. The human ear is sensitive to every minute pressure difference in the air if they are in the audible frequency range. It can detect pressure difference of less than one billionth of atmospheric pressure

Cells make up the smallest level of a living organism such as yourself and other living things. The cellular level of an organism is where the metabolic processes occur that keep the organism alive. That is why the cell is called the fundamental unit of life.

Cells fall into two major categories: eukaryotes and prokaryotes.

Eukaryotes are organisms that contain chromosomes, including plants and animals, as well as fungi (like mushrooms), protozoa, and most algae. Eukaryotes have the following characteristics:

• They have a nucleus that stores their genetic information.

• Animal cells have an organelle called a mitochondria that effectively combines oxygen and food to convert energy to a useable form.

• Plant cells have chloroplasts<i>,</i> which use energy from sunlight to create food for the plant.

• Eukaryotic cells have internal membranes, which create compartments inside the cells that have different functions.

• Plants cells have a cell membrane and a cell wall, which is rigid; animal cells have only a cell membrane, which is soft.

• The cytoskeleton, which reinforces the cytoplasm of the cell, controls cellular movements.

Prokaryotes are cellular organisms that do not have a “true” nucleus. A <i>nucleus</i> is the control center of a cell. A nucleus contains the genetic material packed into chromosomes, and it is associated with other organelles that function in the production of amino acids and proteins based on what the genetic material dictates. Prokaryotes have some genetic material, but it is not as well organized as it is in eukaryotes. Still, prokaryotes are able to reproduce. Examples of these organisms include bacteria and blue-green algae.

A toxin or other foreign substance which induces an immune response in the body, especially the production of antibodies.

When someone speaks, tree leaves rustle, a telephone rings or anything else creates a 'sound', vibrations are sent through the air in all directions. We know them as sound waves.

Almost all sound waves are unique. That's why each person or thing sounds different - and why one person or thing doesn't always sound the same. Some sound waves might be high pitched or low pitched, loud or soft.

When our ears capture sound waves, they convert them into messages our brains can understand. How well they are captured - and how clearly they are sent to our brains - depends on how well our ears work.

The sections of the ear

There are three major parts to the ear:

1. outer ear - catches the sound waves and directs them into the middle ear.
2. middle ear - the middle ear transfers sound waves in air into mechanical pressure waves that are then transferred to the fluids of the inner ear.
3. inner ear (cochlea) - turns pressure waves into sound signals that our brain can understand.

4. To hear naturally, each part needs to work properly.

   How natural hearing works

   

1. Sounds enter the ear canal
   Sound waves move through the ear canal and strike the eardrum.
2. The ear drum and bones of hearing vibrate
   These sound waves cause the eardrum, and the three bones (ossicles) within the middle ear, to vibrate.
3. Fluid moves through the inner ear
   The vibrations move through the fluid in the spiral shaped inner ear – known as the cochlea – and cause the tiny hair cells in the cochlea to move. The hair cells detect the movement and change it into the chemical signals for the hearing nerve.
4. Hearing nerves communicate to the brain
   The hearing nerve then sends the information to the brain with electrical impulses, where they are interpreted as sound.