Describe the two-dimensional figures that result from slicing three-dimensional figures. Here's one lesson, and here's another. Here's some practice problems for your students. Here are some additional interesting questions.

It would be nice to have a program where kids could cut these shapes and see their cross sections.

Slicing a rectangular pyramid by Khan Academy

Cross sections of 3D objects (basic) by Khan Academy

Next, give your students these challenges:

- Tic Tac Toe by NRICH
- Sending Cards by NRICH

Conclude by leading this investigation:

Spectral Blaster Hit (symmetry, patterns)

by MathPickle

7.G.A.3: Describe the two-dimensional figures that result from slicing three-dimensional figures, as in plane sections of right rectangular prisms and right rectangular pyramids.