how can a rhombus be asquare?

when all angles are 90 degrees

All the angles are 90°.

Pretend

A rhombus is a square if the angles are right angles. So, you can look at a square as a rhombus that is also a rectangle. (...or a rectangle that is also a rhombus.)

A right angle rhombus is nothing but a square.

Because the definition of a rhombus is a "quadrilateral all of whose sides have the same length" - notice how it makes no references to the angles.

If the angles at the corners are 90 degrees then you get a square, if they're something else you don't.

A rhombus is a quadrilateral with 4 congruent sides. The only thing keeping it from being a square is that the angles aren't required to be 90°. If you were to take a rhombus with 90° angles, *then* it would be a square.

Equivalently, you could require the diagonals to be congruent and that would also make it fit the criteria of a square.

Answer:
Make the angles all be 90°.
Require the diagonals to be congruent.

It's can be a square if all sides are equidistant. Think of it. It's just a square rotated at its center 45 degrees.