What do the question-mark blocks do in Level 20?

A jump that hits one changes it into a plain framed block and shows a star effect.

When should I go to the door?

After the overhead question block and both question blocks in the row have all changed to plain blocks, return to the floor and walk right.

Mr Potato level guide

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Use the center plain block to reach the upper and row question blocks, turn all three solid, then walk right to the door.

Layout: Mr Potato starts on a flat floor below a row of five framed blocks: plain, question mark, plain, question mark, plain. A separate question-mark block floats above the middle, and the arched door is at the far right.
Goal: Convert the three question-mark blocks into plain blocks, then reach the arched door on the right.
Opening: Move under the middle of the row and jump onto the center plain block to use it as the staging point for the higher blocks.
Danger Zone: The question-mark blocks begin incomplete, and missing the center staging block drops the potato back to the floor before the upper interaction is reached.
Mechanics: Hitting a question-mark block changes it into a plain framed block and creates a star effect; the overhead block and both row question blocks must be converted.

Use the center plain block first; it provides the height needed to reach the separate question-mark block above the row.
Confirm that all three question marks have become plain blocks before making the final walk to the door.
If the board feels stuck, look for the color with the cleanest path and use that to regain space.

Move under the middle of the five-block row and jump onto its center plain block.
From the center block, jump up to hit the separate question-mark block overhead.
Continue the jump sequence through the row to hit both question-mark blocks.
Check the star effects and confirm that the overhead block and both row question blocks are now plain.
Drop back to the floor, hold right, and enter the arched door on the far right.

Running to the door before converting the three question-mark blocks.
Skipping the center plain block and trying to reach the overhead question mark directly from the floor.

What do the question-mark blocks do in Level 20?
A jump that hits one changes it into a plain framed block and shows a star effect.
When should I go to the door?
After the overhead question block and both question blocks in the row have all changed to plain blocks, return to the floor and walk right.