Step 1: Leds and Wire..
The led I used were from an old cell phone, red in colour.It was a handfull to desolder but after destroying a couple I managed to end up with some usable ones.The wire is from the inside of a small motor.I used this because it is small enough and shielded with some kind of varnish so there are no short-circuits happening.Some earth magnets,and a button cell 3v battery with a 10 Ohm resistor.
Step 2: Main Body and Circuit
So, I cut out a DVD case to use as a base.For the shape I used a printout of a heart.For the top part I used a cutout of a CD jewel case with the help of a dremel tool,a file and sand paper.Then I soldered and glued the circuit on the dvd case.In the circuit,if I didn't use the isolating paper between the 2 to magnets,the 2 leds on the right would always be on.This way,the one upper magnet is connected to the battery and the lower one to the leds but only when the other piece with the 2 magnets is put together, the circuit closes and the leds light up.
Step 3: Painting,writing and Putting It Together
Next I sprayed the transparent piece from the cd case from the inside,because I wanted to write on the painted side.The black line around is marker.On the back piece, I used 2sided tape to tape small stripes of foam,a bit taller than the magnet's height.
Step 4: Finally...
Then I stuck the top,painted, piece on top of the foam stripes and I end up with this.It lights up only when the 2 pieces are together with the magnets, reveiling the writing,well...almost....