How can you tell plant cells have thickness?

I just asked my czn she said that:  Let's take a couple of simple examples.

A chicken egg (unfertilized) is a single cell. It has three dimensions.

If you look at microscope slides of tissues, you can move the objective lens up and down and see the inner details of the cells come into and out of focus. That is, even very small cells have a thrid dimension.

Finally, all objects made of matter have three dimensions, not just two.