What the acorn said when it grew up, in a classic math joke

The answer to this crossword question is “GEOMETRY”. This classic math joke involves the phrase “I’m a tree” which is what the acorn says when it grows up. This is a play on words as “tree” sounds like “three” which is a number commonly associated with geometry. Geometry is a branch of mathematics that deals with the study of shapes, sizes, positions, and dimensions of objects in space. It is a fundamental subject in mathematics and has many practical applications in fields such as engineering, architecture, and physics.