Once when Kumāra Kassapa is meditating in Andha,vana, a non-returner brahma, who was his companion in the time of Kassapa Buddha, appears before him, and tells him a riddle of ten questions, which only the Buddha could answer... After reflecting
on its teachings, Kumāra Kassapa becomes an arhat.
... [some interesting background info not in the sutta text itself]
During the time of Kassapa Buddha (the Buddha just before ours), when his teaching was declining, Kumāra Kassapa, together with six other monks, vowed to live a life of rigorous asceticism on a mountain-top to attain liberation. On reaching the summit with the help of a ladder, they threw it off, and began to meditate. The eldest attained arhathood in three days, and the second attained non-return, but the re-
maining five died of starvation on the seventh day without any attainment.
These five companions were Pukkusāti, Bāhiya Dārucīriya, Dabba Malla,putta, the wanderer Sabhiya and Kumāra Kassapa. It was the non-returner brahma who appeared before Kumāra Kassapa and gave him the riddle. This same brahma also suggested to the wanderer Bāhiya to meet the Buddha.
(excerpted from Piya Tan's comy. here )
mn 23 tags: vammika ant hill audtip.org