Pet fish are clever enough to be trained (see links).
This toy would be a small, very lightweight toy bicycle, with large handlebars, and wheels that roll freely and are wide enough for the bike to balance upright.
At the bottom of the fish tank is a flat oval-shaped surface looking not unlike
If you carefully train your goldfish, it will learn to 'ride' the bike around the track by pushing the handlebars. Your goldfish will do so happily, knowing it will be rewarded.
If you have a large enough aquarium and several fish, you might even be able to train them to race each other.
You can then show off your bicycle-riding fish to anyone who visits, particularly radical feminists.
Over three thousand spectators had arrived to see the opening race in this year's series. The crowd was abuzz with excitement as they waited for the competitors to appear.
The lights dimmed, except for the tank in the centre of the arena, which was well-lit but empty. It was roughly sigma-shaped - a large oval with a small tail at one end. Video cameras broadcast pictures of it onto the big screens at each end of the arena.
As the crowd grew silent, expectant, the MC paused for effect before giving them what they wanted, by introducing the first competitor, 'Billy the Fish'.
Cheers rose from all quarters as Billy's trainer brought him on-stage, in a bowl. The trainer first placed a small toy bike in Billy's lane in the main tank, and then scooped Billy out and reverently dropped him into the tank too.
Billy wasn't fazed by the transparent glass walls beside him, or the plastic gate in front of him, enclosing him in a small box on the left-hand side of the tank. He'd been trained for this, and he knew what he had to do, although he'd not raced at this top level before. He flexed his fins in anticipation as he swam in slow circles to warm up.
Soon the other competitors were waiting in their lanes. The crowd roared when they got their first glimpse of last year's champion, 'The Motorpike' (named not for his species but for his menacing looks and powerful speed). Each had a slightly different design of bike to the others, according to the trainers' preferences, and many were custom-designed, though all were within regulation size and weight limits.
All the fish were focused now, resting on top of their bikes, tails gently swishing, poised for the off.
Last-minute bets changed hands amongst the crowd, and then the gates were raised. The fish swam out, pushing their bikes, jostling for position. The race was on!
And five minutes later, after ten laps of the track, the race was won. Not by The Motorpike, but by - yes! Billy the Fish!
The crowd rose to their feet to salute their new hero, while Billy rose to the surface to claim his well-earned fish snacks. The 2015 Tour de Tanks was well and truly underway!