Learn Russian: Горячий металл лежит на весах, бери его щипцами! - The hot metal is lying on the scales, grab it with the tongs!

The word for "scales" is весы. Just like in English, it only exists in the plural form (a plural-only noun). Because it follows the preposition на to indicate a location, it must be in the prepositional case. The prepositional plural ending for nouns ending in -ы is -ах, giving us весах.

What case is щипцами, and why?

It is in the instrumental case. The dictionary form is щипцы (tongs), which is another plural-only noun. We use the instrumental case without a preposition here because the tongs are the physical tool being used to perform the action. For plural-only nouns ending in -ы or -и, the instrumental ending is usually -ами, making it щипцами.

Why does the sentence use его when talking about metal?

In Russian, pronouns must match the grammatical gender of the noun they replace. The word металл is a masculine noun. Therefore, to say "grab it" as a direct object, you must use the masculine accusative pronoun его rather than a neuter pronoun.

Why use the imperfective imperative бери instead of the perfective возьми?

While the perfective возьми is standard for a one-time completed action, the imperfective бери is often used in spoken Russian to urge someone to take something that is right in front of them. It has a prompt, encouraging feel, like saying "go ahead and grab it right now!"