What occurs when two or more chemicals react with each other?

Reactions happen when two or more molecules touch and the molecules change . Bonds are then broken down and created to form new molecules ,Sometimes we will use our chemistry equipment we could see the molecules change ,  the plug and unplug then connectors that represent the chemical bonds

In conclusion : A reaction happens when two or more chemicals mix -

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tracks, burrows, eggshells, nests, tooth marks, or gastroliths

while cooking a piece of 1/2 pound beef steak, a cast iron pan (2.4kg in weight) was heated 260 celsius.