Neither is better. The difference is that "into" conveys a direction of motion (into the box) whereas "in" only conveys position (inside the box). Your choice depends which of these ideas you wish to express. It is similar to the difference between "to" and "towards". Consider, for example, the sentences: "Jack threw the ball to Bill" "Jack threw the ball towards Bill". The first only indicates the ball's destination. The second indicates the direction of motion, not explicitly stating that Bill is the intended recipient.

In everyday speech, I believe both are used interchangeably.

Both sound good to me! They sound a little bit simple, so another option with slightly more advanced vocabulary is: You can put the gift in the red box, because red signifies luck. Another option is: You can put the gift in the red box, because red is a lucky colour.

Option 2 is better:
"You can put the gift in a red box because red means luck." Why? * "In" vs. "Into": * "In" is used when something is already inside or being placed inside without emphasis on movement. * "Into" focuses on the motion or transition of putting something inside. * In this sentence, "in" is more natural because the emphasis is on the box being red and symbolic, not the action of placing the gift. Option 1 isn't wrong but sounds slightly less smooth and natural in everyday usage.

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