List two sets of possible side lengths for two triangles, one set that forms a triangle and one set that does not.

In any triangle, the sum of lengths of any two sides must be greater than the third side.

(1) If we chose three side lengths to be 5 cm, 7 cm and 10 cm. Let's do the check:
5+ 7 = 12 > 10
5+10 = 15 > 7
7 + 10 = 17 > 5
The sum of any two sides is greater than the third, therefore, these three side lengths can form a triangle.

(2) If we chose three side lengths to be 3 cm, 4 cm and 8 cm.Let's do the check:
3 + 4 = 7 which is not greater than 8
The condition is not fulfilled, therefore, these three side lengths cannot form a triangle.