ELI5: Explain Like I'm 5

3-manifolds

Before we learn about 3-manifolds, let's talk about manifolds first. A manifold is like a type of shape that is smooth everywhere. Imagine a globe, it is a type of shape, but it has bumps and dents, so it is not a manifold. In contrast, a balloon is a shape that is smooth everywhere, without bumps or dents, so it is a manifold.

Now, a 3-manifold is a type of manifold that has three dimensions. We can imagine it as a shape that has length, width, and height, like a box or a ball. However, not all 3-dimensional shapes are 3-manifolds. Some shapes have holes or edges, which make them not smooth everywhere, and therefore, not 3-manifolds.

What's interesting about 3-manifolds is that they can have different shapes and structures, like a Rubik's cube or a knot. You can think of it like a puzzle that you can explore and figure out its shape and properties.

Scientists and mathematicians study 3-manifolds to understand more about geometry and topology, which are areas of mathematics that deal with shapes and their properties. They use complicated formulas and algorithms to describe 3-manifolds and their unique features.

In summary, a 3-manifold is a type of smooth shape that has three dimensions, without edges or holes, and it can have different shapes and structures. Scientists and mathematicians study 3-manifolds to learn more about geometry and topology.