hello developers ..

i want to make effect like this effect in this video
this is the link

http://www.youtube.com/watch?v=DVxvlMtjjKI

effects appears at 3:04 to 3:08

i mean that creating hole in the ground and insert something inside it

with respect

The grate would be made of two images. The standard one and a smaller image which is just the middle part. Then it's a matter of stacking your images with different Z-axis numbers.
Background with grate.
Key which drops to the grate.
Little middle image of the grate on top.

When the key image moves down, it looks like it is going inside the grate where it actually goes under another image which has transparent bits.

Also it could be made by using 3 pictures, 2 that represent one half each of the hole, one to appear in front of the magnet and one in the back, then the key which is placed behind both

thank you @WauloK,@Mitaten for this clear answers