I scaled your forehead, and all I saw was 1000.
Large Forehead
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Explanation
Experiment
Explain Bear
Listen up, genius. The joke is that the user is implying your forehead is so large, it looks like a mountain that needs to be "scaled." Then they follow it up with "all I saw was 1000," implying there is a large number in the forehead as a rating. You have a big head!