Animation questions

I have a few questions regarding animation and a little modeling. Any help would be much appreciated.

How can you adjust an object to enter the scene at a certain frame?

How do you change an objects shape as it goes through frames? Example, a box at frame 1 and it turns into a sphere at frame 50.

Also, how would you make the leaves of a tree? Without python, how can you do this?

Thanks

Not sure how/if you can make an object enter a scene at a specified frame, though you can key-frame which layer an object is on. Essentially, making it visible at an arbitrary frame.

Shape keys will let you change a cube into a sphere or vise-versa.

Leaves on a tree? I’d just make a poly plane, divide it down the middle, then divide it 4 or 5 times along it’s length. Adjust the verts to give the shape you need, extrude it for thickness, then either manually, or with the aid of a lattice, bend it into shape lengthways and width-ways. Neal Hirsig has a good video tute on using a lattice to bend stuff.