Literally, rizq refers to that by which some form of benefit is acquired. Sayyiduna Dawood Alayhis Salaam recited the following in one of his supplications:

يَارَازِقَ البُغَاثِ في عَشِّه

“O the Sustainer of the baby crow in its nest!”

When the egg of the crow breaks open, and the baby emerges, the colour of the baby is white. The mother dislikes the child and flies away, leaving the child. Soon flies and mosquitoes come and sit on the baby crow. These become the sustenance of the baby crow. Eventually, hair starts growing on the baby and the black colour becomes manifest. The mother returns, and seeing the colour, becomes attached to the child and begins nurturing the baby.

Allaamah Khattaabi Rahimahullaah said, “Ar-Razzaaq is that Being who has taken the responsibility of sustenance and who tends to every soul with that nourishment which will keep it in existence. His Mercy and Sustenance encompasses all of creation. He does not single out only some for sustenance (e.g. only to a believer and not to a disbeliever or only to a friend and not to an enemy.) Allaah Jalla Jalaalahoo Sends Sustenance to a weak person who has no skill and no means of livelihood, as He does  to a strong and able person. Allaah Jalla Jalaalahoo states:

وَ كَاَيِّنْ مِنْ دَابَّةٍ لَا تَحْمِلُ رِزْقَهَا اللهُ يَرْزُقُهَا وَ اِيَّاكُمْ

How many creatures there are which cannot carry their sustenance.  Allaah Jalla Jalaalahoo sustains them and you.
(Ankaboot)

Ar-Razzaaq  Grants two types of sustenance:

1. One type is that given to the people of the world by Allaah Jalla Jalaalahoo by means of His Servants and Messengers. This is also of two known types:

i) one of the heart which is knowledge and imaan, and
ii) the other is sustenance provided for their bodies. This is halaal sustenance and our Rabb is Ar-Razzaaaq is extremely Kind on those who perform acts of righteousness.

2. The second type is transporting food to the limbs and into its passages, according to a set measure. This can be from halaal or haraam, both of these are sustenance. Allaah Jalla Jalaalahoo is everyone’s Sustainer in this regard. He Grants to all, without specification.

While reciting the Beautiful Names of Allaah Jalla Jalaalahoo, an aarif (one who has recognized Allaah Jalla Jalaalahoo) spontaneously uttered :

منذ عرفت خالقى ما شككت فى رزقي

From the time I recognized my Creator, I never had doubt and concern over my sustenance.

Allaah Jalla Jalaalahoo has taken the responsibility of providing sustenance to man, predators, birds, insects and land and water animals, as He has mentioned,

وَ مَا مِنْ دَابَّةٍ فِى الْاَرْضِ اِلَّا عَلى اللهِ رِزْقُهَا وَ يَعْلَمُ مُسْتَقَرَّهَا وَ مُسْتَوْدَعَهَا كُلٌّ فِيْ كِتَابٍ مُبِيْنٍ

"And there is no living creature on the surface of the earth except that Allaah Jalla Jalaalahoo has taken responsibility for its sustenance. (For sending of sustenance, knowledge is necessary). He Knows their permanent and temporary residence.”
(There, He Bestows their sustenance to them. And even though everything is in His Knowledge, together with this,) everything is written in an open book (i.e. Lowhe Mahfooz).
(Hud)

When we invite 100 people to our house. As a precautionary measure, we prepare for 110 people. Then why, (Allaah forbid!) do we regard the system of Allaah Jalla Jalaalahoo so weak! If He is able to Arrange for the creation of 100,000 new-born babies, can He not make arrangements for their food ? He has created hunger and He has created the system of satiating hunger. The child has not yet come out of the womb of the mother, and the blood is already present for the formation of food. When a child grows bigger, it is granted the inclination of crying for its sustenance. 

Ar-Razzaaq Grants milk teeth to this infant, which are temporary. As the child grows, these milk teeth begin to fall off, and are gradually replaced by permanent teeth. The milk teeth number twenty, five on each upper and lower side, right, and left. These teeth remain till the age of six. During this time, molars are formed, bringing the total number to twenty four. Thereafter, the front teeth fall off, and are replaced. As time progresses, the number of teeth total twenty eight. At the age of 17 or 18, the wisdom teeth emerge, bringing the total number of teeth to thirty-two.

As food enters the stomach, the acid therein destroys the different types of germs present in the food. This acid is so powerful that it digests food, but not the stomach itself.The stomach lining is coated with a sticky, glutinous layer, which does not allow the acid penetrate. Within the stomach there are approximately 350 million glands, which secrete this  special chemical material.

Ar-Razzaaq has Created sustenance in the air for the birds, and in the water for aquatic animals.  The camel resides in such places where there is not an abundance of water and vegetation. Allaah Jalla Jalaalahoo has Placed a cache of water and fat in its body, so that at the time of necessity, it can be of assistance. In fact, every animal’s body fat has been created such, that if food is unavailable, it can be used as a source of food, thus conserving the muscles.