Show an ordinary Bic-style pen. Talk about psychokinesis and people who bend metal objects by sheer mental concentration. Start to rub the pen. It becomes flexible, bending like rubber! No optical illusion here. The pen actually bends! Rub the pen one more time and it is again solid! Immediately hand the pen to a spectator for thorough examination! They can even write with it! Amazing.

Only one pen is used.
Nothing is added or taken away.
No sleight of hand.
The pen can be used for writing.
Totally examinable.
Instant reset.