Invisible Sketch

First, you can simply draw with this app.

Set the line thickness with the bottom left slider.

Select a color using the bottom right slider.

To delete the last changes, repeatedly press the button with a cross.

To delete a drawing, press the same button and hold it for 2 seconds.

Second, you can use this app to play.

Move the top left slider all the way to the left.

Ask a friend to look at the screen while you draw or write on it.

Then ask your friend to guess what you drew.

To see your drawing, move the slider to the right.

Third, you can draw over pictures.

Press the left button, and then press any picture in your phone's gallery.

Move the picture to the desired position and scale it.

Press the second button from the right.

Move the top right slider to the left so that the image becomes pale and draw over it.