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Or feed the marker's liquid reservoir from a larger tank.

Or use a chalkboard.

Or some gears and a battery.

Chalk would still run out and need to be replaced. Some kind of sand drawing would be more of an infinite "ink".

Hilariously, a good way to do this with infinite "ink" would be to draw the letters on a touch screen with a stylus.

Or, just use one of the children's magnetic drawing board?