Hi guys,
I am an artist I fell in love with arduino , it has been the one of the best thing that I found in life, I feel like the time that I find out about Linux, Macs and cms.
any way.
I want to use arduino in an art installation.
I wanto to have up to 13 spots ( grid ) that will will have small scluptures,
so when some body place them on the grid ( from 2 to 7 sculptures at the time ) the ardruino will detect the presence of the scupltures and will triger a video projection ( from a pool of files- movies- quartz composer patches etc.)
any ideas in how can this be done.
I want to use a mac mini, and quartz composer
suggestions are so welcome 
You need sensors of some sort, possibly IR, ultra sound or pressure.
If the sculptures are on pedestals you could put pressure or peizo sensors on the pedestal so when a sculpture is placed on it, it's triggered.
You might also consider using some computer vision algorithms to detect objects on the grid. This would need a webcam (or something like it) and not use the arduino.
You have to decide if the Arduino has to know what sculpture has been placed on a spot or only if a sculpture has been placed on a spot.
If it is the former then the best bet is to use some form of RFID in the base of each sculpture with readers on the spots. However this is expensive.
If it is the latter then probably the best would be to include a small magnet in the base of each sculpture and have the spots contain a reed switch or hall effect sensor. The down side of this is that the placing would have to be accurate or you would have to have several magnets to allow some latitude in placement.
There are other forms of sensor you could use like a reflective IR sensor and just have foil on the base. This would give a large latitude in placement. But I would steer clear from peizo sensors as these only give you a signal when placing or removing a sculpture. That is they only give a signal on force change not absolute force.
Hope that gives you enough to go at.
To build on Grumpy_Mike's ideas, you could do a "poor man's RFID" to identify each statue by having a small array of sensors/pads per pedestal, and an encoded combination of magnets/foil/contacts in the foot of each statue.
If you're going to go with magnets, using the bipolar nature of magnets to your advantage. that is using just 4 magnets you can get 16 unique combinations (++++, ----, +-+-, --+-, etc).
using the bipolar nature of magnets to your advantage
Just a point, that means using hall effect sensors not read switches. Also you need the right sort of sensor. Some hall effect switches / sensors don't care about the polarity of the field and others do. If you are going for this make sure you pick the right sort.