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The difference is chiefly in the environment where you would use the one and the other. "Sieh mal" is the more formal expression. "Guck mal" or "kuck mal" is the more colloquial one.

Let's try to make a list of similar expressions, ordered by degressive level of being formal (or ascending level of being colloquial):

Würden Sie Ihre Aufmerksamkeit einmal dorthin wenden?

Sehen Sie dort!

Sehen Sie mal!

Schauen Sie!

Sieh mal!

Kuck mal!

Guck mal!

Da, guck!

(There might be a certain degree of personal discretion here. Other German native speakers might put them in slightly different order.)

The difference is chiefly in the environment where you would use the one and the other. "Sieh mal" is the more formal expression. "Guck mal" or "kuck mal" is the more colloquial one.

The difference is chiefly in the environment where you would use the one and the other. "Sieh mal" is the more formal expression. "Guck mal" or "kuck mal" is the more colloquial one.

Let's try to make a list of similar expressions, ordered by degressive level of being formal (or ascending level of being colloquial):

Würden Sie Ihre Aufmerksamkeit einmal dorthin wenden?

Sehen Sie dort!

Sehen Sie mal!

Schauen Sie!

Sieh mal!

Kuck mal!

Guck mal!

Da, guck!

(There might be a certain degree of personal discretion here. Other German native speakers might put them in slightly different order.)

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The difference is chiefly in the environment where you would use the one and the other. "Sieh mal" is the more formal expression. "Guck mal" or "kuck mal" is the more colloquial one.