This "Let's Investigate!" problem is suited for everyone from Reception upwards to KS 1 and 2 and beyond.

Start off younger and less mathematically able children with a few different strips, rods, straws or (better still) pieces from a class construction set. If you do not have access to a construction set, strips can be made from card cut to length - but remember to leave a bit extra on the ends, so that they can joined with push-through brass paper fasteners or brads.

There is no need to use all the different lengths at the beginning - perhaps, start with just four and add others as children find they have exhausted the possibilities with the limited number of strips.

The children should be led to discover which combinations cannot be made and why, as well as to express how rigid triangles are.

The more able children should be encouraged to find a way to prove that they have all discovered all of the possible triangles that meet the criteria.