While most of the coverage of Sunday's stage in the Tour de France was about the "tack attack", the outcome of that day's race was ultimately decided by rheology. Gel rheology in particular. The gel is barely covered by the cycling media, so let's get to that first:
While the main pack of riders was dealing with the tacks, a smaller group was well up the road, and one of the riders in that group was the feared Slovakian Peter Sagan. Sagan has already one three stages of the race this year and looked to be on the way to his fourth since nobody in the breakaway group could outsprint him unless Sagan stopped in the sprint to tie his shoes, and even that might not be enough of a hindrance. He is that fast.
But Sagan made one mistake with only 11.5 kilometers to go in the day's race, a mistake that was the equivalent of stopping to tie his shows: He was hungry and did something about it. He reached in his back pocket and pulled out an energy gel [*]. At that point, another one of the riders in the group, Leon Sanchez, attacked and rode as hard as he could. He was able to open up a gap and keep it all the way to the finish. Sagan ended up coming in second.
So what does rheology have to do with this? Well, Sagan had three options to address his hunger - drink something, eat something or go for a gel. Had he gone for his water bottle (assuming that it contained some kind of energy in it and wasn't just water), he would have been able to respond to Sanchez's attack. Eating something was not an option as chewing solid food is normally only done in slow portions of the race. You Mom always told you to not talk with food in your mouth and the same rule holds for bicycling racing - it's impossible to ride hard with a mouth full of food. So that left the gel as the only option. The fact that the gel is soft enough that it can be quickly swallowed, but still stiff enough that you need a good hand grip to extrude it is what caused Sagan to choose that option, and thereby lose the race.
Years ago before these gels existed, Sagan would not have had the gel option and most likely would have won the race, assuming he didn't "bonk" (hit the wall/run out of energy), but that sweet gel with that easy to swallow rheology tempted him and he paid the price.
Don't feel too bad for Sagan. He's only 22 and just starting his career. There will be many more wins to come, but hopefully his coach will speak to him about different rheological options for refueling. (If not, I'd be willing to be hired on as a rheology consultant!)
[*] These gels are carbohydrate-based, with various sugars, electrolytes... You tear off the top of the package and squeeze the gel out into your mouth.