try this

sit down.
place your hands on your lap, fingers pointing out.
Both feet flat on the ground.

Tap your hand = use base of palm as pivot
tap your foot = use heel as pivot

Tap your right hand and left foot at the same time. When you do this, your left hand and right foot should be pointing up.
Then tap your left hand and right foot, when you do this, your right hand and left foot should be pointing up.
Repeat as many times as you can.

Note: the taps by hand and foot MUST be synchronized.

Once you can do it 10 times in a row, do it AS FAST AS YOU CAN 10 times in a row.

Pretty challenging >.<