A building block set having a plurality of building blocks (20, 40, 60, 80, 100), with at least some of the blocks (20, 40, 60, 80, 100) comprising: a generally transparent body (21, 41, 61, 81, 101) having first and second openings and an internal light source (35, 55, 75, 95, 115) operable into an illuminated state in response to an electrical input; a male electrically conductive connector (29, 49, 69, 89, 109); a female electrically conductive connector (32, 52, 72, 92, 112) fitted into the second opening of the transparent body (21, 41, 61, 81, 101); and circuit means (36, 56, 76, 116) within the transparent body (21, 41, 61, 81, 101) electrically coupled with the internal light source (35, 55, 75, 95, 115) and both the male and female connectors (29, 49, 69, 89, 109, 32, 52, 72, 92, 112) for powering the internal light source (35, 55, 75, 95, 115) in response to an electrical input into just one connector (29, 49, 69, 89, 109, 32, 52, 72, 92, 112); the male electrically conductive connector (29, 49, 69, 89, 109) being fitted into the first opening of the transparent body (21, 41, 61, 81, 101); characterised in that: the transparent body (21, 41, 61, 81, 101) further comprises a hollow barrel (26, 46, 66, 86, 106) extending outwardly to the first opening; the second opening is dimensioned to receive a barrel (26, 46, 66, 86, 106) from a second block (20, 40, 60, 80, 100) so that a male electrically conductive connector (29, 49, 69, 89, 109) of the second block (20, 40, 60, 80, 100) can be mechanically interconnected with the female connector (32, 52, 72, 92, 112); and the barrel (26, 46, 66, 86, 106) surrounds the male connector (29, 49, 69, 89, 109) such that, when the barrel (26, 46, 66, 86, 106) of the second block (20, 40, 60, 80, 100) is located in the second opening, the barrel (26, 46, 66, 86, 106) is interposed between the female connector (32, 52, 72, 92, 112) and the transparent body (21, 41, 61, 81, 101) and the male connector (29, 49, 69, 89, 109) of the second block (20, 40, 60, 80, 100) is located inside the female connector (32, 52, 72, 92, 112). a generally transparent body (21, 41, 61, 81, 101) having first and second openings and an internal light source (35, 55, 75, 95, 115) operable into an illuminated state in response to an electrical input; a male electrically conductive connector (29, 49, 69, 89, 109); a female electrically conductive connector (32, 52, 72, 92, 112) fitted into the second opening of the transparent body (21, 41, 61, 81, 101); and circuit means (36, 56, 76, 116) within the transparent body (21, 41, 61, 81, 101) electrically coupled with the internal light source (35, 55, 75, 95, 115) and both the male and female connectors (29, 49, 69, 89, 109, 32, 52, 72, 92, 112) for powering the internal light source (35, 55, 75, 95, 115) in response to an electrical input into just one connector (29, 49, 69, 89, 109, 32, 52, 72, 92, 112); the male electrically conductive connector (29, 49, 69, 89, 109) being fitted into the first opening of the transparent body (21, 41, 61, 81, 101); characterised in that: the transparent body (21, 41, 61, 81, 101) further comprises a hollow barrel (26, 46, 66, 86, 106) extending outwardly to the first opening; the second opening is dimensioned to receive a barrel (26, 46, 66, 86, 106) from a second block (20, 40, 60, 80, 100) so that a male electrically conductive connector (29, 49, 69, 89, 109) of the second block (20, 40, 60, 80, 100) can be mechanically interconnected with the female connector (32, 52, 72, 92, 112); and the barrel (26, 46, 66, 86, 106) surrounds the male connector (29, 49, 69, 89, 109) such that, when the barrel (26, 46, 66, 86, 106) of the second block (20, 40, 60, 80, 100) is located in the second opening, the barrel (26, 46, 66, 86, 106) is interposed between the female connector (32, 52, 72, 92, 112) and the transparent body (21, 41, 61, 81, 101) and the male connector (29, 49, 69, 89, 109) of the second block (20, 40, 60, 80, 100) is located inside the female connector (32, 52, 72, 92, 112). the transparent body (21, 41, 61, 81, 101) further comprises a hollow barrel (26, 46, 66, 86, 106) extending outwardly to the first opening; the second opening is dimensioned to receive a barrel (26, 46, 66, 86, 106) from a second block (20, 40, 60, 80, 100) so that a male electrically conductive connector (29, 49, 69, 89, 109) of the second block (20, 40, 60, 80, 100) can be mechanically interconnected with the female connector (32, 52, 72, 92, 112); and the barrel (26, 46, 66, 86, 106) surrounds the male connector (29, 49, 69, 89, 109) such that, when the barrel (26, 46, 66, 86, 106) of the second block (20, 40, 60, 80, 100) is located in the second opening, the barrel (26, 46, 66, 86, 106) is interposed between the female connector (32, 52, 72, 92, 112) and the transparent body (21, 41, 61, 81, 101) and the male connector (29, 49, 69, 89, 109) of the second block (20, 40, 60, 80, 100) is located inside the female connector (32, 52, 72, 92, 112).