You're asking a great question! However, there isn't one single substance that melts at exactly 20 degrees Celsius (68 degrees Fahrenheit). Here's why and how to approach this:

* Melting Point Varies: The melting point of a substance is a specific temperature at which it transitions from a solid to a liquid. This temperature depends heavily on the substance's chemical composition and its molecular structure.

* Many Possibilities: There are many different substances that could melt around 20 degrees Celsius.

* Examples:

* Some Organic Compounds: Certain fats, waxes, and some organic chemicals have melting points in this range.

* Mixtures: A mixture of different substances could melt around 20 degrees Celsius.

To figure out what specific substance melts at 20 degrees, you'd need more information. For example:

* What is the substance's chemical formula?

* What is its appearance (solid, liquid, gas, color)?

* What is its purpose or use?

Let me know if you have any additional details about the substance you're interested in!