Funds for pensions are usually mix of stocks and bonds, but stocks are really bad for defined benefit contributions because in a downturn you will have to overs

Funds for pensions are usually mix of stocks and bonds, but stocks are really bad for defined benefit contributions because in a downturn you will have to oversell. This is something that actually happened in 2008 and broke some pension funds in the U.S. So bonds would not have that issue (treasury bonds, low-risk bonds) so you have no volatility. But even they will not produce enough for the benefits that are promised: as soon as someone outlives their contribution the fund might not be actuarially safe. It turns out it is exceedingly difficult to mitigate all risk in a retirement fund, and government makes the promise that if they fail, they will pass the bill to someone else. Pension funds are truly evil.