だんだん and 少しずつ: gradual change and small increments
Compare a sky growing gradually darker with adding water little by little, separating a changing state from incremental amounts.
A state that changes gradually
だんだん means gradually. 空がだんだん暗くなりました。 says that the sky grew gradually darker.
練習して、だんだん上手になりました。 says that practice led to gradual improvement. No fixed numerical gain for each occasion is specified.
Dividing an amount into small portions
少しずつ, read すこしずつ, means little by little. 少しずつ水を加えてください。 asks for water to be added in small amounts.
毎日、少しずつ本を読みます。 says that the person reads a little of a book each day. It gives no exact page count.
An input and the resulting change
少しずつ水を加えると、色がだんだん薄くなります。 says that adding water little by little makes the color gradually paler. 少しずつ describes the added amounts; だんだん describes the change.
少しずつ does not always indicate an increase. 荷物を少しずつ減らしました。 says that luggage was reduced little by little. 減らす supplies the direction of change.
Which expression divides the added water into portions in 少しずつ水を加えると、色がだんだん薄くなります。?
Show the answer and why
少しずつ describes adding small amounts. だんだん describes the color’s gradual change toward a paler state.