My 9 year old is in 3rd grade and he really struggles with math. I have never been a math scholar myself. I did OK, but never great and I got a lot of help from my dad who is a math wiz. My son only has me to help him and I am no math wiz. Since my son struggles I know that what might be obvious to me may not be obvious to him and I have to be very careful and clear about how I explain things to him, and many times I have to change my thinking to re-explain things to him in a way that he understands. His class just finished a unit on shapes. 2-Dimensional and 3-Dimensional Shapes to be exact. For those of you who have 3rd graders or any grade level of children studying these shapes I am going to try to simplify the explanation for you.
2 Dimensional Shapes
A 2-Dimensional shape is by definition: A shape whose points are all in one plane. OK most kids would hear that and go "WHAT?" Basically it means a shape that is flat. So while the shape will have length (the measurement side to side) and width (the measurement from top to bottom) it will not have any thickness. It is flat. So all of your basic shapes including your squares, triangles, rectangles and even circles are all 2 dimensional shapes.
Think about a regular 2D movie, it is shown on a flat screen, the characters, nor the scenery, nor the props pop out at you. In your home or at school a piece of paper would be considered a 2 D shape because it has length and width but it is flat it has no thickness. A poster is another good example, even a towel, or a flat bed sheet would be examples of 2-Dimensional shapes.
Pretty easy huh?
Now 3-Dimensional Shapes are a little bit more difficult to explain and a little more interesting to look at. A 3-Dimensional shape is a shape that does not lie flat. Like a 2-Dimensional shape it has length, and it has width, but it also has thickness. A pyramid is a very good example of a 3 dimensional shape.
A 3-Dimensional shape has a base that it stands from. For instance the base of a pyramid is a square, but up from that square comes the triangles, these triangles all come together at a point and give the shape it's thickness making it a pyramid. Just to make things a little more confusing though a 3-Demensional shape can also have curved surfaces. In the case of curved surfaces at all points and any given distance it is all the same from the center point. This is called a sphere. Balls are actually spheres. They are a circular shape with thickness.A cone is another example of a 3 dimensional shape. It has a round base with a curved surface and one vertex, which is called the apex. Don't let the word vertex scare you, it is just a fancy way of saying the point where everything comes together. So in a cone the circular part is the base and the bottom tip is the vertex or apex.A cylinder is a 3 dimensional shape that has two circular bases. The circular bases are parallel to one another which means they are the same distance apart but will never touch. The cylinder also has curved surfaces that touch each other at all points. A good example of s cylinder would be a plumbing pipe or a toilet paper roll.
For other examples of the 3-Dimensional shapes see the picture above showing several examples of shapes.
So if your child's teacher asks them to tell them the base of a 3-Demensional shape they are asking the shape of which the shape starts from or would sit on.