From BlenderWiki

Build Modifier

Description

The Build modifier causes the faces of the mesh object to appear, one after the other, over time. If the material of the mesh is a halo rather than a standard one, then the vertices of the mesh, not the faces, appear one after another.

By default, faces (or vertices) appear in the order in which they are stored in memory (by default, they order of creation). These orders can be altered by selecting the object and…

• For vertices, in Edit mode, using the Xsort button (Mesh Tools panel) to reorder them along the X-axis (TODO: Once the new modelling chapter is published, add a link to Doc:Manual/Modeling/Meshes/Editing/Misc#Vertex List Ordering --Mont29 15:51, 5 February 2010 (UTC)).
• For faces, in Object mode, pressing CtrlF to sort them, based on different elements (TODO: Once the new modelling chapter is published, add a link to Doc:Manual/Modeling/Meshes/Editing/Misc#Face List Ordering --Mont29 15:51, 5 February 2010 (UTC)).

Never press the Apply button in this modifier panel if you want to keep the build animation!

Options

Start

The start frame of the building process.

Length

The number of frames over which to build up.

Randomize

Randomizes the order that the faces are built up.

Seed

The random seed. Change this to get a different “random” order of appearing – this order being always the same for a given (seed, object) set.

Example

Imagine a city being built right before your eyes. How about a fountain of halos?

This example shows a simple tube, beauty short subdivided a few times, with the modifier turned on, from frames 1 to 100 (in steps of 10), with no additional animation.