The Facts About Income Tax Millions faithfully file their 1040 forms each April. But some things about federal income taxes may surprise you. Share | Related Content Comparing Investments This calculator compares the net gain of a taxable investment versus a tax-favored one. Red Flags for Tax Auditors Here are six flags that may make your tax return prime for an IRS audit. Tax Freedom Day Assess how many days you'll work to pay your federal tax liability.