how to start an animation by clicking on an object

hi - newbie so thanks for all your help.

I am a teacher that would rather make use of keynote rather than powerpoint - this enables me to put any presentations on to an iPad ( the iPad is projected on to the whiteboard) thus enabling me to pass the iPad around the room making my presentations interactive.

In powerpoint I am able to click on an object to start an animation - for example I have square boxes that cover answers - when the box is clicked it fades away to reveal the answer below.

In keynote the only function for animation that I can work out is that I can get animation to run in a sequence/order not by selection.

If anybody has got the answer to this it would be very helpful.

It also appears that the powerpoint version for mac doesn't have this feature???

I have tried exporting the keynote presentation to powerpoint apply the transition then try and open it back up in keynote but with no success. (this did work with powerpoint for mac - but I can't open those files on my iPad)

Thanks