Stephen Omandi scratched out the number "55" on the sign advertising buckets of maize and wrote in the new price: 60 Kenya shillings.
The price hike amounted to only $0.06. But for the residents of Nairobi's largest slum, where most people live on $1 a day, that 10 percent increase is enough to make the essential food stuff unaffordable.
"We haven't gotten many customers because they complain, 'Why have you increased the price?'" said Omandi. "Five shillings. It's a lot of money, because many people could not afford it at 55, and now it's 60."
Food prices are rising across the globe, driven in part by the higher transport costs that accompany rising oil prices. The World Bank said last week that food prices are 36 percent higher today than a year ago, and are pushing people "deeper into poverty."
But no region has been hit harder by rising food costs than Africa over the last three months. Wheat costs 87 percent more in Sudan. Rice is up 30 percent in Chad. Maize has risen at least 25 percent in Uganda, Somalia, Mozambique and Kenya.
Omandi used to sell 40 small buckets of maize a day, but on one recent day _ the first of his most recent price hike _ he sold only two. Omandi was forced to increase his price because the government had just raised the price ceiling it sets for gasoline. The whole cycle made customers grumble.
"They said, 'Have you increased it again?' It used to be 35 Kenya shillings late last year. Now it has increased almost 100 percent," Omandi said.
About 100 people blocked traffic near parliament in downtown Nairobi on Tuesday to protest the price increases. A day earlier the government cut taxes on kerosene and diesel, but protesters said the cuts were too small. Yash Pal Ghai, a constitutional law expert who took part in the demonstration, said the issue was both prices and corruption.
"The revenue authority said recently that one-third of the (tax) revenue is stolen by politicians, bureaucrats and businessmen," he said. "Some people have a single meal a day while others live in obscene luxury and comfort. It is amazing there has not been a rebellion by now."
Some parliamentarians were heckled as they were driven in luxury vehicles into parliament, where a debate on food costs was held. One demonstrator held a sign that read: "Parliamentarians are just filling their potbellies while the common citizen is getting thinner."
In Uganda, Kenya's western neighbor, the country's top opposition politician has led three marches over the last 10 days to protest higher food and fuel prices. Police have unleashed tear gas and bullets on the protests, and even shot the opposition leader, Kizza Besigye, in the hand. Protests have been held countrywide.
Thomas Mugisha, a worker in a mattress factory in Kampala, Uganda's capital, said he now walks to work because the price of public transport rose from $0.30 to $0.60. The protest walks have been a reflection of that reality.
"We decided to walk to places of work as a sign of solidarity with other many Ugandans who are suffering from high prices," said Alice Alaso, an opposition parliamentarian who invoked the example of the French Revolution during an interview. She complained of high military spending at a time when people can't even afford food.
"Our government has become insensitive toward its people's suffering," she said.
The price of maize in Uganda has risen 114 percent over the last year, according to the World Bank. That's the highest year-over-year increase in the world. Gasoline and meat prices are also soaring.
"We used to eat meat three times a week, but now we eat beans due to the high price of meat," said Zaida Namuli, 35, a Kampala resident.
An elementary school teacher, Silvia Acha, said she pays twice as much for rice as she used to, but that her $85 monthly salary has remained unchanged. "The government should come to our aid," she said.
The World Bank said much of the recent increase of food prices was due to a 21 percent rise in oil prices in the first quarter, which increased because of unrest in the Middle East and North Africa. Higher crude oil prices mean products like corn and vegetable oil are more frequently used as biofuels. Transportation costs rise.
"More poor people are suffering and more people could become poor because of high and volatile food prices," said World Bank Group President Robert B. Zoellick. "We have to put food first and protect the poor and vulnerable, who spend most of their money on food."
In Nairobi's largest slum, Kibera, many people eat what more affluent Kenyans simply don't want: Dried mini sardines, cow lungs, and fish heads discarded from higher-end shops and restaurants.
Many of the slum's youngest are fed by the World Food Program or aid groups. First Love, a U.S. group, feeds 1,150 students a day in Kibera. Breakfast is a cup of porridge.
"Kids will sneak an extra cup and take it home. We're OK with that because we know that will be his dinner," said the group's Philip Muthui, who was fed by the program himself when he was a student.
In Kibera's skinny dirt lanes, where goods are transported by wheelbarrow, Benjamin Mwalepe sells buckets of charcoal for $0.30. Mwalepe, who cares for six children, some his nieces and nephews, noted that the price of a jerry can of potable water has risen from $0.03 two months ago to $0.05 today.
When asked what costs more today, he answered "everything." But there is no more money, so there's only one solution.
"You have to eat a smaller portion," he said.
Associated Press reporter Godfrey Olukya in Kampala, Uganda contributed to this report.