Limit definition of continuity at a point x = a:
If the limit as x approaches a from the left equals the limit as x approaches a from the right and that value is equal to f(a), f is continuous at x =a.
So, for y = |x| at x=0:
limit as x approaches zero from the left is 0.
limit as x approaches zero from the right is also 0.
f(0)=0
Therefore, y=|x| is continuous at (0,0).