First Tribute in Falling Sand, Day 7

The first tribute in the Falling Sand on Day 7 will run into a sphinx with a riddle carved into a stone tablet around the sphinx's neck. The tribute must accurately solve the riddle; if the tribute does not, the sphinx will attack.

Your ancestor knows me,
Grandfather shows me,
Though each man will fight me,
none wins.

Mountains ignore me,
But none can outscore me,
None know when infinite me
begins.