adding dynamic fields to a model ...

Hi, here i describe my problem:

mi application must allow 2 basic issues:

first: the users may be able to create 'people' (this would be a model with a scaffold, and is not an issue, the problem comes with requierement #2)

second: the users should be able to add new fields to that model, for example: the model 'person' may have 2 fields, let say: 'name' and 'age', the basic idea is to give the user the ability to create new fields like 'sex' and 'occupation' if he like to do so, i wouldn't like to do ALTER TABLE to solve this .... any idea will be most welcome ... thanks