"He lost five kilograms."

Translation: 他瘦了五公斤。

[NessaNessaJoy]

Why is a kilo a 公斤？ What is the etymology, etc?

[LeiFeiRalf]

Jin is a measure that's about half a kilo and probably similar to the pound (lb) in English. Think of gongjin as a long pound or kilo. Literally i guess it's the public weight measure.

My teacher told me that they tend to use jin for small amounts and gongjin for more than a couple.

[Gabrielle145359]

斤 is an old Chinese weight or mass measuring unit. 公 means "public".
公斤 Etymologically is like "the public weight".
公 is used for metric measurements and 英 （short for England) is used for standard/imlerial measurements.

[KX3.]

又称“千克”（2018年后可有改变）

[CinnamonTe1]

Can someone explain why fat/plump (胖) and bored (闷) are listed as adjectives but they are being used as verbs in this skill?

I thought nouns and adjectives needed a 很 between them?

[mendosi]

In this example sentence, the 了 is used in a way that indicates a change of state. 他很瘦 = He is skinny. 他瘦了 = He became skinny.

See also: https://resources.allsetlearning.com/chinese/grammar/Change_of_state_with_%22le%22

There are also other ways that you can apply an adjective to a noun with intensifiers which mean that you don't separate the noun and adjective with 很. 例如：他瘦極了 (他瘦极了) = He is extremely skinny. (This is introduced in Food 3, by the way.)