袋 and 箱: containers and layers of packaging
Distinguish bags from boxes and track an inner container against its outer packaging.
Putting something in a bag
袋, read ふくろ, is a bag or pouch used to hold things. 紙の袋にパンを入れました。 means putting bread in a paper bag.
袋の口を閉じました。 means closing the opening of the bag. 口 here refers to that opening, not a person's mouth.
Putting something in a box
箱, read はこ, means box. 箱の中に本が三冊あります。 says that there are three books inside a box.
箱にふたをしました。 means putting a lid on a box. It does not specify the contents or whether the box was locked.
Inner and outer packaging
箱を袋に入れました。 means putting a box into a bag. 箱を marks what was inserted, and 袋に marks the destination.
袋から箱を取り出しました。 means taking a box out of a bag. Identifying the source marked by から keeps the packaging layers in the right order.
What is the outer container in 箱を袋に入れました?
Show the answer and why
The bag. The box went inside it.